 Section 1 of the Atomic Bombings of Hiroshima and Nagasaki. This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, visit LibriVox.org. Reading by Dennis Sayers on the 63rd anniversary of the events. The Atomic Bombings of Hiroshima and Nagasaki. By the Manhattan Engineer District, June 29, 1946. Section 1. Including the forward introduction. The Manhattan Project Investigating Group. Propaganda Summary of Damages and Injuries and Main Conclusions. Forward. This report describes the effects of the atomic bombs which were dropped on the Japanese cities of Hiroshima and Nagasaki on August 6th and 9th, 1945, respectively. It summarizes all the authentic information that is available on damage to structures, injuries to personnel, morale effect, etc., which can be released at this time without prejudicing the security of the United States. This report has been compiled by the Manhattan Engineer District of the United States Army under the direction of Major General Leslie R. Groves. Special acknowledgement to those whose work contributed largely to this report is made to the Special Manhattan Engineer District Investigating Group, the United States Strategic Bombing Survey, the British Mission to Japan, and the Joint Atomic Bomb Investigating Group Medical, and particularly to the following individuals. Colonel Stafford L. Warren, Medical Corps, United States Army for his evaluation of medical data. Captain Henry L. Barnett, Medical Corps, United States Army for his evaluation of medical data. Dr. R. Serber, for his comments on flash burn. Dr. Hans Beth, Cornell University for his information on the nature of atomic explosions. Majors Nolan Varley and Walter C. Youngs, Corps of Engineer, United States Army for their evaluation of physical damage to structures. J. O. Hirschfelder, J. L. McGee, M. Hall, and S. T. Cohen of the Los Alamos Laboratory for their data on nuclear explosions. Lieutenant Colonel David B. Parker, Corps of Engineers, United States Army, for editing this report. Introduction. Statement by the President of the United States, quote, 16 hours ago an American airplane dropped one bomb on Hiroshima, Japan, and destroyed its usefulness to the enemy. That bomb had more power than 20,000 tons of TNT. It had more than 2,000 times the blast power of the British Grand Slam, which is the largest bomb ever yet used in the history of warfare. Close, quote. These fateful words of the President on August 6, 1945 marked the first public announcement of the greatest scientific achievement in history. The atomic bomb, first tested in New Mexico on July 16, 1945, had just been used against a military target. On August 6, 1945, at 8.15 a.m. Japanese time, a B-29 heavy bomber flying at high altitude dropped the first atomic bomb on Hiroshima. More than four square miles of the city were instantly and completely devastated. 66,000 people were killed and 69,000 injured. On August 9, three days later, at 11.02 a.m., another B-29 dropped the second bomb on the industrial section of the city of Nagasaki, totally destroying one and one-half square miles of the city, killing 39,000 persons, and injuring 25,000 more. On August 10, the day after the atomic bombing of Nagasaki, the Japanese government requested that it be permitted to surrender under the terms of the Potsdam Declaration of July 26, which it had previously ignored. The Manhattan Project Atomic Bomb Investigating Group. On August 11, 1945, two days after the bombing of Nagasaki, a message was dispatched from Major General Leslie R. Groves to Brigadier General Thomas F. Farrell, who was his deputy in atomic bomb work, and was representing him in operations in the Pacific, directing him to organize a special Manhattan Project Atomic Bomb Investigating Group. This group was to secure scientific, technical, and medical intelligence in the atomic bomb field from within Japan as soon as possible after the cessation of hostilities. The mission was to consist of three groups. One, group for Hiroshima. Two, group for Nagasaki. Three, group to secure information concerning general Japanese activities in the field of atomic bombs. The first two groups were organized to accompany the first American troops into Hiroshima and Nagasaki. The primary purposes of the mission were as follows in order of importance. One, to make certain that no unusual hazards were present in the bombed cities. Two, to secure all possible information concerning the effects of the bombs, both usual and unusual, and particularly with regard to radioactive effects, if any, on the targets or elsewhere. General Groves further stated that all available specialist personnel and instruments would be sent from the United States and that the Supreme Allied Commander in the Pacific would be informed about the organization of the mission. On the same day, the 11th of August, the special personnel who formed the part of the investigating group to be sent from the United States were selected and ordered to California with instructions to proceed overseas at once to accomplish the purpose set forth in the message to General Ferrell. The main party departed from Hamilton Field, California, on the morning of 13 August and arrived in the Marianas on 15 August. On 12 August, the Chief of Staff sent the Theater Commander the following message. Quote, for MacArthur signed Marshall. Groves has ordered Ferrell at Tennian to organize a scientific group of three sections for potential use in Japan, if such use should be desired. The first group is for Hiroshima, the second for Nagasaki, and the third for the purpose of securing information concerning general Japanese activities in the field of atomic weapons. The groups for Hiroshima and Nagasaki should enter those cities with the first American troops in order that these troops shall not be subjected to any possible toxic effects, although we have no reason to believe that any such effects actually exist. Ferrell and his organization have all available information on this subject. Close Quote, General Ferrell arrived in Yokohama on 30 August with the commanding general of the Eighth Army. Colonel Warren, who was Chief of the Radiological Division of the District, arrived on 7 September. The main body of the investigating group followed later. Preliminary inspections of Hiroshima and Nagasaki were made on 8, 9, and 13, 14 September respectively. Members of the press had been enabled to proceed General Ferrell to Hiroshima. The special groups spent 16 days in Nagasaki and 4 days in Hiroshima, during which time they collected as much information as was possible under their directives, which called for a prompt report. After General Ferrell returned to the U.S. to make his preliminary report, the groups were headed by Brigadier General J.B. Newman, Jr. More extensive surveys have been made since that time by other agencies, who had more time and personnel available for the purpose, and much of their additional data has thrown further light on the effects of the bombings. This data has been duly considered in the making of this report. Propaganda On the day after the Hiroshima strike, General Ferrell received instructions from the War Department to engage in a propaganda campaign against the Japanese Empire in connection with the new weapon and its use against Hiroshima. The campaign was to include leaflets and any other propaganda considered appropriate, with the fullest cooperation of SensaPak of the Navy and the United States Strategic Air Forces. He initiated promptly a campaign which included the preparation and distribution of leaflets, broadcasting via shortwave every 15 minutes over radio Saipan, and the printing at Saipan, and distribution over the Empire. Of a Japanese language newspaper which included the description and photographs of the Hiroshima strike. The campaign proposed, one, dropping 16 million leaflets in a period of nine days on 47 Japanese cities with population of over 100,000. These cities represented more than 40% of the total population. Two, broadcast of propaganda at regular intervals over radio Saipan. Three, distribution of 500,000 Japanese language newspapers containing stories and pictures of the atomic bomb attacks. The campaign continued until the Japanese began their surrender negotiations. At that time, some 6 million leaflets and a large number of newspapers had been dropped. The radio broadcasts in Japanese had been carried out at regular 15 minute intervals. Summary of damages and injuries. Both the Hiroshima and the Nagasaki atomic bombs exhibited similar effects. The damages to man-made structures and other inanimate objects was the result in both cities of the following effects of the explosions. A, blast or pressure wave similar to that of normal explosions. B, primary fires, that is those fires started instantaneously by the heat radiated from the atomic explosion. C, secondary fires, that is those fires resulting from the collapse of buildings, damage to electrical systems, overturning of stoves and other primary effects of the blast. D, spread of the original fires, B and C, to other structures. The casualties sustained by the inhabitants of both cities were due to A, flash burns, caused directly by the almost instantaneous radiation of heat and light at the moment of the explosion. B, burns resulting from the fires caused by the explosion. C, mechanical injuries caused by collapse of buildings, flying debris and forcible hurling about of persons struck by the blast pressure waves. D, radiation injuries caused by the instantaneous penetrating radiation in many respects similar to excessive x-ray exposure from the nuclear explosion. All of these effective radiations occurred during the first minute after initiation of the explosion and nearly all occurred during the first second of the explosion. No casualties were suffered as a result of any persistent radioactivity of fission products of the bomb or any induced radioactivity of objects near the explosion. The gamma radiations emitted by the nuclear explosion did not, of course, inflict any damage on structures. The number of casualties which resulted from the pure blast effect alone, that is because of simple pressure, was probably negligible in comparison to that caused by other effects. The central portions of the cities underneath the explosion suffered almost complete destruction. The only surviving objects were the frames of a small number of strong reinforced concrete buildings which were not collapsed by the blast. Most of these buildings suffered extensive damage from interior fires, had their windows, doors, and partitions knocked out, and all other fixtures which were not integral parts of the reinforced concrete frames burned or blown away. The casualties in such buildings near the center of the explosion were almost 100%. In Hiroshima, fires sprang up simultaneously all over the wide flat central area of the city. These fires soon combined in an immense fire storm. High winds blowing inwards toward the center of a large conflagration, similar to those caused by ordinary mass incendiary raids. The resulting terrific conflagration burned out almost everything which had not already been destroyed by the blast in a roughly circular area of 4.4 square miles around the point directly under the explosion. This point will hereafter, in this report, be referred to as X. Similar fires broke out in Nagasaki, but no devastating firestorm resulted as in Hiroshima because of the irregular shape of the city. In both cities, the blast totally destroyed everything within a radius of a mile from the center of the explosion, except for certain reinforced concrete frames as noted above. The atomic explosion almost completely destroyed Hiroshima's identity as a city. Over a fourth of the population was killed in one stroke, and an additional fourth seriously injured, so that even if there had been no damage to structures and installations, the normal city life would still have been completely shattered. Nearly everything was heavily damaged up to a radius of three miles from the blast, and beyond this distance damage, although comparatively light, extended for several more miles. Glass was broken up to 12 miles. In Nagasaki, a smaller area of the city was actually destroyed than in Hiroshima because the hills which enclosed the target area restricted the spread of the great blast, but careful examination of the effects of the explosion gave evidence of even greater blast effects than in Hiroshima. Total destruction spread over an area of about three square miles. Over a third of the 50,000 buildings in the target area of Nagasaki were destroyed or seriously damaged. The complete destruction of the huge steel works and the torpedo plant was especially impressive. The steel frames of all buildings within a mile of the explosion were pushed away as by a giant hand from the point of detonation. The badly burned area extended for three miles in length. The hillsides up to a radius of 8,000 feet were scorched, giving them an autumnal appearance. Main conclusions. The following are the main conclusions which were reached after thorough examination of the effects of the bombs dropped on Hiroshima and Nagasaki. One, no harmful amounts of persistent radioactivity were present after the explosions as determined by A. Measurements of the intensity of radioactivity at the time of investigation and B, failure to find any clinical evidence of persons harmed by persistent radioactivity. The effects of the atomic bombs on human beings were of three main types. A, burns. Remarkable for one, the great ground area over which they were inflicted, and two, the prevalence of flash burns caused by the instantaneous heat radiation. B, mechanical injuries, also remarkable for the wide area in which suffered. C, effects resulting from penetrating gamma radiation. The effects from radiation were due to instantaneous discharge of radiation at the moment of explosion and not to persistent radioactivity of either fission products or other substances whose radioactivity might have been induced by proximity to the explosions. The effects of the atomic bombs on structures and installations were of two types. A, destruction caused by the great pressure from the blast. And B, destruction caused by the fires either started directly by the great heat radiation or indirectly through the collapse of buildings, wiring, etc. 4. The actual tonnage of TNT which would have caused the same blast damage was approximately of the order of 20,000 tons. 5. In respect to their height of burst, the bombs performed exactly according to design. 6. The bombs were placed in such positions that they could not have done more damage from any alternative bursting point in either city. 7. The heights of burst were correctly chosen having regard to the type of destruction it was desired to cause. 8. The information collected would enable a reasonably accurate prediction to be made of the blast damage likely to be caused in any city where an atomic explosion could be affected. End of Section 1 of the Atomic Bombing of Hiroshima and Nagasaki, Section 2 of the Atomic Bombings of Hiroshima and Nagasaki. This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, visit LibriVox.org. The Atomic Bombings of Hiroshima and Nagasaki by the Manhattan Engineer District, June 29, 1946, Section 2, including the selection of the target, description of the cities before the bombings, Hiroshima, Nagasaki, the attacks, Hiroshima, Nagasaki, general comparison of Hiroshima and Nagasaki. 7. The selection of the target Some of the most frequent queries concerning the atomic bombs are those dealing with the selection of the targets and the decision as to when the bombs would be used. The approximate date for the first use of the bomb was set in the fall of 1942 after the Army had taken over the direction of and responsibility for the atomic bomb project. At that time, under the scientific assumptions which turned out to be correct, the summer of 1945 was named as the most likely date when sufficient production would have been achieved to make it possible, actually, to construct and utilize an atomic bomb. It was essential before this time to develop the technique of constructing and detonating the bomb and to make an almost infinite number of scientific and engineering developments and tests. Between the fall of 1942 and June 1945, the estimated probabilities of success had risen from about 60% to above 90%. However, not until July 16, 1945, when the first full-scale test took place in New Mexico, was it conclusively proven that the theories, calculations, and engineering were correct and that the bomb would be successful. The test in New Mexico was held six days after sufficient material had become available for the first bomb. The Hiroshima bomb was ready, awaiting suitable weather on July 31, and the Nagasaki bomb was used as soon after the Hiroshima bomb as it was practicable to operate the second mission. The work on the actual selection of targets for the atomic bomb was begun in the spring of 1945. This was done in close cooperation with the commanding general, Army Air Forces, and his headquarters. A number of experts in various fields assisted in the study. These included mathematicians, theoretical physicists, experts on the blast effects of bombs, weather consultants, and various other specialists. Some of the important considerations were, A. The range of the aircraft which would carry the bomb, B. The desirability of visual bombing in order to ensure the most effective use of the bomb, C. Probable weather conditions in the target areas, D. Importance of having one primary and two secondary targets for each mission, so that if weather conditions prohibited bombing the target, there would be at least two alternates, E. Selection of targets to produce the greatest military effect on the Japanese people, and thereby most effectively shortened the war, F. The morale effect upon the enemy. These led in turn to the following, A. Since the atomic bomb was expected to produce its greatest amount of damage by primary blast effect and next greatest by fires, the targets should contain a large percentage of closely built frame buildings and other construction that would be most susceptible to damage by blast and fire. B. The maximum blast effect of the bomb was calculated to extend over an area of approximately one mile in radius, therefore the selected targets should contain a densely built up area of at least this size. C. The selected targets should have a high military strategic value. D. The first target should be relatively untouched by previous bombing in order that the effect of a single atomic bomb could be determined. D. The weather records showed that for five years there had never been two successive good visual bombing days over Tokyo, indicating what might be expected over other targets in the home islands. The worst month of the year for visual bombing was believed to be June, after which the weather should improve slightly during July and August, and then become worse again during September. Since good bombing conditions would occur rarely, the most intense plans and preparations were necessary in order to secure accurate weather forecasts and to arrange for full utilization of whatever good weather might occur. It was also very desirable to start the raids before September. Description of the cities before the bombings. Hiroshima. The city of Hiroshima is located on the broad, flat delta of the Ota River, which has seven channel outlets dividing the city into six islands which project into Hiroshima Bay. The city is almost entirely flat and only slightly above sea level. To the northwest and northeast of the city, some hills rise to 700 feet. A single hill in the eastern part of the city proper, about one half mile long and 221 feet in height, interrupted to some extent the spreading of the blast damage. Otherwise the city was fully exposed to the bomb. Of a city area of over 26 square miles, only seven square miles were completely built up. There was no marked separation of commercial, industrial, and residential zones. 75% of the population was concentrated in the densely built up area in the center of the city. Hiroshima was a city of considerable military importance. It contained the 2nd Army Headquarters, which commanded the defense of all of southern Japan. The city was a communications center, a storage point, and an assembly area for troops. To quote a Japanese report, quote, probably more than a thousand times since the beginning of the war did the Hiroshima citizens see off with cries of banzai, the troops leaving from the harbor, close quote. The center of the city contained a number of reinforced concrete buildings as well as lighter structures. Outside the center the area was congested by a dense collection of small wooden workshops set among Japanese houses. A few larger industrial plants lay near the outskirts of the city. The houses were of wooden construction with tile roofs. Many of the industrial buildings also were of wood frame construction. The city, as a whole, was highly susceptible to fire damage. Some of the reinforced concrete buildings were of a far stronger construction than is required by normal standards in America because of the earthquake danger in Japan. This exceptionally strong construction undoubtedly accounted for the fact that the framework of some of the buildings which were fairly close to the center of damage in the city did not collapse. The population of Hiroshima had reached a peak of over 380,000 earlier in the war, but prior to the atomic bombing the population had steadily decreased because of a systematic evacuation ordered by the Japanese government. At the time of the attack the population was approximately 255,000. This figure is based on the registered population used by the Japanese in computing ration quantities and the estimates of additional workers and troops who were brought into the city may not be highly accurate. Hiroshima thus had approximately the same number of people as the city of Providence, Rhode Island, or Dallas, Texas. Nagasaki. Nagasaki lies at the head of a long bay which forms the best natural harbor on the southern Japanese home island of Kyushu. The main commercial and residential area of the city lies on a small plain near the end of the bay. Two rivers divided by a mountain spur form the two main valleys in which the city lies. This mountain spur and the irregular layout of the city tremendously reduced the area of destruction so that at first glance Nagasaki appeared to have been less devastated than Hiroshima. The heavily built up area of the city is confined by the terrain to less than four square miles out of a total of about 35 square miles in the city as a whole. The city of Nagasaki had been one of the largest seaports in southern Japan and was of great wartime importance because of its many and varied industries, including the production of ordnance, ships, military equipment, and other war materials. The narrow long strip attacked was of particular importance because of its industries. In contrast to many modern aspects of Nagasaki, the residences almost without exception were of flimsy typical Japanese construction consisting of wood or wood frame buildings with wood walls with or without plaster and tile roofs. Many of the smaller industries and business establishments were also housed in wooden buildings or flimsily built masonry buildings. Nagasaki had been permitted to grow for many years without conforming to any definite city zoning plan and therefore residences were constructed adjacent to factory buildings and to each other almost as close as it was possible to build them throughout the entire industrial valley. The Attacks Hiroshima Hiroshima was the primary target of the first atomic bomb mission. The mission went smoothly in every respect. The weather was good and the crew and equipment functioned perfectly. In every detail the attack was carried out exactly as planned and the bomb performed exactly as expected. The bomb exploded over Hiroshima at 8.15 on the morning of August 6, 1945. About an hour previously the Japanese early warning radar net had detected the approach of some American aircraft headed for the southern part of Japan. The alert had been given and radio broadcasting stopped in many cities among them Hiroshima. The planes approached the coast at a very high altitude. At nearly 8 a.m. the radar operator in Hiroshima determined that the number of planes coming in was very small, probably not more than three, and the air raid alert was lifted. The normal radio broadcast warning was given to the people that it might be advisable to go to shelter if B-29s were actually sighted, but no raid was expected beyond some sort of reconnaissance. At 8.15 a.m. the bomb exploded with a blinding flash in the sky and a great rush of air and a loud rumble of noise extended for many miles around the city. The first blast was soon followed by the sounds of falling buildings and of growing fires, and a great cloud of dust and smoke began to cast a pall of darkness over the city. At 8.16 a.m. the Tokyo control operator of the Japanese Broadcasting Corporation noticed that the Hiroshima station had gone off the air. He tried to use another telephone line to re-establish his program, but it too had failed. About twenty minutes later the Tokyo Railroad Telegraph Center realized that the main line telegraph had stopped working just north of Hiroshima. From some small railway stops within ten miles of the city there came unofficial and confused reports of a terrible explosion in Hiroshima. All these reports were transmitted to the headquarters of the Japanese general staff. Military headquarters repeatedly tried to call the Army Control Station in Hiroshima. The complete silence from that city puzzled the men at headquarters. They knew that no large enemy raid could have occurred, and they knew that no sizable store of explosives was in Hiroshima at that time. A young officer of the Japanese general staff was instructed to fly immediately to Hiroshima, to land, survey the damage, and return to Tokyo with reliable information for the staff. It was generally felt at headquarters that nothing serious had taken place, that it was all a terrible rumor starting from a few sparks of truth. The staff officer went to the airport and took off for the southwest. After flying for about three hours, while still nearly one hundred miles from Hiroshima, he and his pilot saw a great cloud of smoke from the bomb. In the bright afternoon the remains of Hiroshima were burning. Their plane soon reached the city around which they circled in disbelief. A great scar on the land still burning and covered by a heavy cloud of smoke was all that was left of a great city. They landed south of the city, and the staff officer immediately began to organize relief measures after reporting to Tokyo. Tokyo's first knowledge of what had really caused the disaster came from the White House, public announcement in Washington, 16 hours after Hiroshima had been hit by the atomic bomb. Nagasaki had never been subjected to large-scale bombing prior to the explosion of the atomic bomb there. On August 1, 1945, however, a number of high-explosive bombs were dropped on the city. A few of these bombs hit in the shipyards and dock areas in the southwest portion of the city. Several of the bombs hit the Mitsubishi Steel and Arms Works, and six bombs landed at the Nagasaki Medical School and Hospital, with three direct hits on buildings there. While the damage from these few bombs were relatively small, it created considerable concern in Nagasaki, and a number of people, principally school children, were evacuated to rural areas for safety, thus reducing the population in the city at the time of the atomic attack. On the morning of August 9, 1945, at about 7.50 a.m. Japanese time, an air raid alert was sounded in Nagasaki, but the all-clear signal was given at 8.30. When only two B-29 Superfortresses were sighted at 10.53, the Japanese apparently assumed that the planes were only on reconnaissance, and no further alarm was given. A few moments later, at 11 o'clock, the observation B-29 dropped instruments attached to three parachutes, and at 11.02 the other plane released the atomic bomb. The bomb exploded high over the industrial valley of Nagasaki, almost midway between the Mitsubishi Steel and Arms Works in the south, and the Mitsubishi Urukami Ordnance Works, Torpedo Works, in the north, the two principal targets of the city. Despite its extreme importance, the first bombing mission on Hiroshima had been almost routine. The second mission was not so uneventful. Again, the crew was specially trained and selected, but bad weather introduced some momentous complications. These complications are best described in the brief account of the mission's Weponeer Commander, now Captain, F. L. Ashworth, U.S.N., who was in technical command of the bomb and was charged with the responsibility of ensuring that the bomb was successfully dropped at the proper time and on the designated target. His narrative runs as follows. The night of our takeoff was one of tropical rain squalls and flashes of lightning stabbed into the darkness with disconcerting regularity. The weather forecast told us of storms all the way from the Marianas to the Empire. Our rendezvous was to be off the southeast coast of Kyushu, some 1,500 miles away. There we were to join with our two companion Observation B-29s that took off a few minutes behind us. Skillful piloting and expert navigation brought us to the rendezvous without incident. About five minutes after our arrival, we were joined by the first of our B-29s. The second, however, failed to arrive, having apparently been thrown off its course by storms during the night. We waited 30 minutes and then proceeded without the second plane toward the target area. During the approach to the target, the special instruments installed in the plane told us that the bomb was ready to function. We were prepared to drop the second atomic bomb on Japan. But fate was against us, for the target was completely obscured by smoke and haze. Three times we attempted bombing runs, but without success. Then, with anti-aircraft fire bursting around us and with a number of enemy fighters coming up after us, we headed for our secondary target, Nagasaki. The bomb burst with a blinding flash, and a huge column of black smoke swirled up toward us. Out of this column of smoke, there boiled a great swirling mushroom of gray smoke, luminous with red flashing flame that reached to 40,000 feet in less than eight minutes. Below through the clouds we could see the pall of black smoke ringed with fire that covered what had been the industrial area of Nagasaki. By this time our fuel supply was dangerously low, so after one quick circle of Nagasaki, we headed direct for Okinawa for an emergency landing and refueling. General comparison of Hiroshima and Nagasaki. It was not at first apparent to even trained observers visiting the two Japanese cities which of the two bombs had been the most effective. In some respects Hiroshima looked worse than Nagasaki. The fire damage in Hiroshima was much more complete. The center of the city was hit and everything but the reinforced concrete buildings had virtually disappeared. A desert of clear swept charred remains with only a few strong building frames left standing was a terrifying sight. At Nagasaki there were no buildings just underneath the center of explosion. The damage to the Mitsubishi armworks and the torpedo works was spectacular, but not overwhelming. There was something left to see and the main contours of some of the buildings were still normal. An observer could stand in the center of Hiroshima and get a view of most of the city. The hills prevented a similar overall view in Nagasaki. Hiroshima impressed itself on one's mind as a vast expanse of desolation, but nothing as vivid was left in one's memory of Nagasaki. When the observers began to note details, however, striking differences appeared. Trees were down in both cities, but the large trees which fell in Hiroshima were uprooted while those in Nagasaki were actually snapped off. A few reinforced concrete buildings were smashed at the center in Hiroshima, but in Nagasaki equally heavy damage could be found 2,300 feet from X. In the study of objects which gave definite clues to the blast pressure, such as squashed tin cans, dished metal plates, bent or snapped poles, and like, it was soon evident that the Nagasaki bomb had been much more effective than the Hiroshima bomb. In the description of damage which follows, it will be noted that the radius for the amount of damage was greater in Nagasaki than Hiroshima. End of Section 2 of the Atomic Bombings of Hiroshima and Nagasaki. Section 3 of the Atomic Bombings of Hiroshima and Nagasaki. This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, visit LibriVox.org. The Atomic Bombings of Hiroshima and Nagasaki by the Manhattan Engineer District. June 29, 1946. Section 3. General Description of the Damage Caused by the Atomic Explosions In considering the devastation in the two cities, it should be remembered that the city's differences in shape and topography resulted in great differences in the damages. Hiroshima was all on low flat ground and was roughly circular in shape. Nagasaki was much cut up by hills and mountain spurs, with no regularity to its shape. In Hiroshima, almost everything up to about one mile from X was completely destroyed, except for a small number, about fifty, of heavily reinforced concrete buildings, most of which were specially designed to withstand earthquake shock, which were not collapsed by the blast. Most of these buildings had their interiors completely gutted and all windows, doors, sashes, and frames ripped out. In Nagasaki, nearly everything within one half mile of the explosion was destroyed, including heavy structures. All Japanese homes were destroyed within one and one half miles from X. Underground air raid shelters with earth cover roofs immediately below the explosion had their roofs caved in, but beyond one half mile from X they suffered no damage. In Nagasaki, fifteen hundred feet from X, high quality steel frame buildings were not completely collapsed, but the entire building suffered mass distortion and all panels and roofs were blown in. In Nagasaki, two thousand feet from X reinforced concrete buildings with ten inch walls and six inch floors were collapsed. Reinforced concrete buildings with four inch walls and roofs were standing, but were badly damaged. At two thousand feet some nine inch concrete walls were completely destroyed. In Nagasaki, three thousand five hundred feet from X, church buildings with eighteen inch brick walls were completely destroyed. Twelve inch brick walls were severely cracked as far as five thousand feet. In Hiroshima, four thousand four hundred feet from X, multi-story brick buildings were completely demolished. In Nagasaki, similar buildings were destroyed to five thousand three hundred feet. In Hiroshima, roof tiles were bubbled, melted, by the flash heat out to four thousand feet from X. In Nagasaki, the same effect was observed to six thousand five hundred feet. In Hiroshima, steel frame buildings were destroyed four thousand two hundred feet from X and to four thousand eight hundred feet in Nagasaki. In both cities, the mass distortion of large steel buildings was observed out to four thousand five hundred feet from X. In Nagasaki, reinforced concrete smoke stacks with eight inch walls, specially designed to withstand earthquake shocks, were overturned up to four thousand feet from X. In Hiroshima, steel frame buildings suffered severe structural damage up to five thousand seven hundred feet from X and in Nagasaki the same damage was sustained as far as six thousand feet. In Nagasaki, nine inch brick walls were heavily cracked to five thousand feet, were moderately cracked to six thousand feet and slightly cracked to eight thousand feet. In both cities, light concrete buildings collapsed out to four thousand seven hundred feet. In Hiroshima, multi-story brick buildings suffered structural damage up to six thousand six hundred feet and in Nagasaki up to six thousand five hundred feet from X. In both cities, overhead electric installations were destroyed up to five thousand five hundred feet and trolley cars were destroyed up to five thousand five hundred feet and damaged to ten thousand five hundred feet. Flash ignition of dry combustible material was observed as far as six thousand four hundred feet from X in Hiroshima and in Nagasaki as far as ten thousand feet from X. Severe damage to gas holders occurred out to six thousand five hundred feet in both cities. All Japanese homes were seriously damaged up to six thousand five hundred feet in Hiroshima and to eight thousand feet in Nagasaki. Most Japanese homes were damaged up to eight thousand feet in Hiroshima and ten thousand five hundred feet in Nagasaki. The hillsides in Nagasaki were scorched by the flash radiation of heat as far as eight thousand feet from X. This scorching gave the hillsides the appearance of premature autumn. In Nagasaki, very heavy plaster damage was observed in many buildings up to nine thousand feet. Moderate damage was sustained as far as twelve thousand feet and light damage up to fifteen thousand feet. The flash charring of wooden telephone poles was observed up to nine thousand five hundred feet from X in Hiroshima and to eleven thousand feet in Nagasaki. Some reports indicate flash burns as far as thirteen thousand feet from X in both places. Severe displacement of roof tiles was observed up to eight thousand feet in Hiroshima and to ten thousand feet in Nagasaki. In Nagasaki, very heavy damage to window frames and doors was observed up to eight thousand feet and light damage up to ten thousand feet. Roof and wall coverings on steel frame buildings were destroyed out to eleven thousand feet. Although the sources of many fires were difficult to trace accurately, it is believed that fires were started by primary heat radiation as far as fifteen thousand feet from X. Roof damage extended as far as sixteen thousand feet from X in Hiroshima and in Nagasaki. The actual collapse of buildings was observed at the extreme range of twenty three thousand feet from X in Nagasaki. Although complete window damage was observed only up to twelve thousand feet from X, some window damage occurred in Nagasaki up to forty thousand feet and actual breakage of glass occurred up to sixty thousand feet. Fire damage was sustained in a circular area in Hiroshima with a mean radius of about six thousand feet and a maximum radius of about eleven thousand feet. Similar heavy damage occurred in Nagasaki south of X up to ten thousand feet where it was stopped on a river course. In Hiroshima over sixty thousand of ninety thousand buildings were destroyed or severely damaged by the atomic bomb. This figure represents over sixty seven percent of the city's structures. In Nagasaki fourteen thousand or twenty seven percent of fifty two thousand residences were completely destroyed and five thousand four hundred or ten percent were half destroyed. Only twelve percent remain undamaged. This destruction was limited by the layout of the city. The following is a summary of the damage to buildings in Nagasaki as determined from a ground survey made by the Japanese. Destruction of buildings and houses. Number. Percentage. Compiled by Nagasaki municipality. Total in Nagasaki before atomic explosion. Fifty thousand or one hundred percent blasted not burned two thousand six hundred and fifty two or five point three percent blasted and burned eleven thousand four hundred ninety four twenty three percent blasted and or burned fourteen thousand one hundred forty six twenty eight point three percent partially burned or blasted five thousand four hundred and forty one ten point nine percent total buildings and houses destroyed nineteen thousand five hundred eighty seven thirty nine point two percent undamaged thirty thousand four hundred and thirteen sixty point eight percent in Hiroshima all utilities and transportation services were disrupted for varying lengths of time in general however services were restored about as rapidly as they could be used by the depleted population through railroad service was in order in Hiroshima on eight august and electric power was available in most of the surviving parts on seven august the day after the bombing the reservoir of the city was not damaged being nearly two miles from x however seventy thousand breaks and water pipes in buildings and dwellings were caused by the blast and fire effects rolling transportation suffered extensive damage the damage to railroad tracks and roads was comparatively small however the electric power transmission and distribution systems were badly wrecked the telephone system was approximately eighty percent damaged and no service was restored until fifteen august despite the customary japanese lack of attention to sanitation measures no major epidemic broke out in the bombed cities although the conditions following the bombings makes this fact seems surprising the experience of other bomb cities in both germany and japan show Hiroshima and Nagasaki not to be isolated cases the atomic explosion over Nagasaki affected an overall area of approximately forty two point nine square miles of which about eight point five square miles were water and only about nine point eight square miles were built up the remainder being partially settled approximately thirty six percent of the built up areas were seriously damaged the area most severely damaged had an average radius of about one mile and covered about two point nine square miles of which two point four were built up in Nagasaki buildings with structural steel frames principally the misubishi plant as far as six thousand feet from x were severely damaged these buildings were typical of wartime meal construction in america and great britain except that some of the frames were somewhat less substantial the damage consisted of windows broken out one hundred percent steel sashes ripped out or bent corrugated metal or corrugated asbestos roofs and sightings ripped off roofs bent or destroyed roof tresses collapsed columns bent and cracked and concrete foundations for columns rotated damage to buildings with structural steel frames was more severe where the buildings received the effect of the blast on their sides than where the blast hit the ends of the buildings because the buildings had more stiffness resistance to negative movement at the top of columns in a longitudinal direction many of the lightly constructed steel frame buildings collapsed completely while some of the heavily constructed to carry the weight of heavy cranes and loads were stripped of roof and sightings but the frames were only partially injured the next most seriously damaged area in Nagasaki lies outside the two point nine square miles just described and embraces approximately four point two square miles of which 29 percent was built up the damage from blast and fire was moderate here but in some sections portions of main business districts many secondary fires started and spread rapidly resulting in about as much overall destruction as in areas much closer to x an area of partial damage by blast and fire lies just outside the one just described and comprises approximately 35.8 square miles of this area roughly one sixth was built up and one quarter was water the extent of damage varied from serious severe damage to roofs and windows in the main business section of Nagasaki two point five miles from x to minor broken or occasionally broken windows at a distance of seven miles southeast of x as intended the bomb was exploded at an almost ideal location over Nagasaki to do the maximum damage to industry including the Mitsubishi steel and armworks the Mitsubishi Urukami ordinance works torpedo works and numerous factories factory training schools and other industrial establishments with a minimum destruction of dwellings and consequently a minimum amount of casualties had the bomb been dropped farther south the Mitsubishi Urukami ordinance works would not have been so severely damaged but the main business and residential districts of Nagasaki would have sustained much greater damage casualties calculations show that the structural steel and reinforced concrete frames which survived the blast fairly close to x could not have withstood the estimated peak pressures developed against the total areas presented by the sides and roof of the buildings the survival of these frames is explained by the fact that they were not actually required to withstand the peak pressure because the windows were quickly knocked out and roof and siding stripped off thereby reducing total area and relieving the pressure while this saved the building frame it permitted severe damage to building interior and contents and injuries to the building occupants buildings without large panel openings through which the pressure could dissipate were completely crushed even when their frames were as strong as those which survived the damage sustained by reinforced concrete buildings depended both on the proximity to x and the type and strength of the reinforced concrete construction some of the buildings with reinforced concrete frames also had reinforced concrete walls ceilings and partitions while others had brick or concrete tile walls covered either with plaster or ornamental stone with partitions of metal glass and plaster with the exception of the nagasaki medical school and hospital group which was designed to withstand earthquakes and was therefore of heavier construction than most american structures most of the reinforced concrete structures could be classified only as fair with concrete of low strength and density with many of the columns beams and slabs under designed and improperly reinforced these facts account for some of the structural failures which occurred in general the atomic bomb explosion damaged all windows and ripped out bent or twisted most of the steel window or door sashes ripped doors from hinges damaged all suspended wood metal and plaster ceilings the blasts concussion also caused great damage to equipment by tumbling and battering fires generally of secondary origin consumed practically all combustible material caused plaster to crack off burned all wooden trim stair covering wooden frames of wooden suspended ceilings beds mattresses and mats and fused glass ruined all equipment not already destroyed by the blast ruined all electrical wiring plumbing and caused spalling of concrete columns and beams and many of the rooms almost without exception masonry buildings of either brick or stone within the effective limits of the blast were severely damaged so that most of them were flattened or reduced to rubble the wreckage of a church approximately 1800 feet east of x and Nagasaki was one of the few masonry buildings still recognizable and only portions of the walls of this structure were left standing these walls were extremely thick about two feet the two domes of the church had reinforced concrete frames and although they were toppled they held together as units practically every wooden building or building with timber frame within 2.0 miles of x was either completely destroyed or very seriously damaged and significant damage in Nagasaki resulted as far as 3 miles from x nearly all such buildings collapsed and a very large number were consumed by fire a reference to the various photographs depicting damage shows that although most of the buildings within the effective limits of the blast were totally destroyed or severely damaged a large number of chimneys even close to x were left standing apparently uninjured by the concussion one explanation is that concrete chimneys are approximately cylindrical in shape and consequently offer much less wind resistance than flat surfaces such as buildings another explanation is that since the cities were subject to typhoons the more modern chimneys were probably designed to withstand winds of high velocity it is also probable that most of the recently constructed chimneys as well as the more modern buildings were constructed to withstand the acceleration of rather severe earthquakes since the bombs were exploded high in the air chimneys relatively close to x were subjected to more of a downward than a lateral pressure and consequently the overturning moment was much less than might have been anticipated although the blasts damaged many bridges to some extent bridge damage was on the whole slight in comparison to that suffered by buildings the damage varied from only damaged railings to complete destruction of the superstructure some of the bridges were wrecked and the spans were shoved off their piers and into the riverbed below by the force of the blast others particularly steel plate girder bridges were badly buckled by the blast pressure none of the failures observed could be attributed to inadequate design or structural weaknesses the roads and railroad and street railway crackage sustained practically no primary damage as a result of the explosion most of the damage to railroads occurred from secondary causes such as fires and damage to bridges or other structures rolling stock as well as automobiles trolleys and buses were destroyed and burned up to a considerable distance from x streets were impassable for a while because of the debris but they were not damaged the height of the bomb explosion probably explains the absence of direct damage to railroads and roads a large part of the electric supply was interrupted by the bomb blast chiefly through damage to electric substations and overhead transmission systems both gas works in Nagasaki were severely damaged by the bomb these works would have required six to seven months to get into operation in addition to the damage sustained by the electrical and gas systems severe damage to the water supply system was reported by the Japanese government the chief damage was a number of breaks in the large water mains and in almost all of the distributing pipes in the areas which were affected by the blast Nagasaki was still suffering from a water shortage inside the city six weeks after the atomic attack the Nagasaki prefectural report describes vividly the effects of the bomb on the city and its inhabitants quote within a radius of one kilometer from x men and animals died almost instantaneously and outside a radius of one kilometer and within a radius of two kilometers from x some men and animals died instantly from the great blast and heat but the great majority were seriously or superficially injured houses and other structures were completely destroyed while fires broke out everywhere trees were uprooted and withered by the heat outside a radius of two kilometers and within a radius of four kilometers from x men and animals suffered various degrees of injury from window glass and other fragments scattered about by the blast and many were burned by the intense heat dwellings and other structures were half damaged by blast outside a radius of four kilometers and within a radius of eight kilometers living creatures were injured by materials blown about by the blast the majority were only superficially wounded houses were only half or partially damaged close quote the british mission to japan interpreted their observations of the destruction of buildings to apply to similar construction of their own as follows a similar bomb exploding in a similar fashion would produce the following effects on normal british houses up to 1 000 yards from x it would cause complete damage up to one mile from x it would damage the houses beyond repair up to 1.5 miles from x it would render them uninhabitable without extensive repair particularly to roof timbers up to 2.5 miles from x it would render them uninhabitable until first aid repairs had been carried out the fire damage in both cities was tremendous but was more complete in Hiroshima than in Nagasaki the effect of the fires was to change profoundly the appearance of the city and to leave the central part bare except for some reinforced concrete and steel frames and objects such as safes chimney stacks and pieces of twisted sheet metal the fire damage resulted more from the properties of the cities themselves than from those of the bombs the conflagration in Hiroshima caused high winds to spring up as air was drawn in toward the center of the burning area creating a firestorm the wind velocity in the city had been less than five miles per hour before the bombing but the fire wind attained a velocity of 30 to 40 miles per hour these great winds restricted the perimeter of the fire but greatly added to the damage of the conflagration within the perimeter and caused the deaths of many persons who might otherwise have escaped in Nagasaki very severe damage was caused by fires but no extensive fire storm engulfed the city in both cities some of the fires close to x were no doubt started by the ignition of highly combustible materials such as paper straw and dry cloth upon the instantaneous radiation of heat from the nuclear explosion the presence of large amounts of unburnt combustible materials near x however indicated that even though the heat of the blast was very intense its duration was insufficient to raise the temperature of many materials to the kenneling point except in cases where conditions were ideal the majority of the fires were of secondary origin starting from the usual electrical short circuits broken gas lines overturned stoves open fires charcoal braziers lamps etc following collapse or serious damage from the direct blast fire fighting and rescue units were stripped of men and equipment almost 30 hours elapsed before any rescue parties were observable in Hiroshima only a handful of fire engines were available for fighting the ensuing fires and none of these were of first class type in any instance however it is not likely that any firefighting equipment or personnel or organization could have affected any significant reduction in the amount of damage caused by the tremendous conflagration a study of numerous aerial photographs made prior to the atomic bombings indicates that between 10 june and 9 august 1945 the japanese constructed fire breaks in certain areas of the cities in order to control large scale fires in general these fire breaks were not effective because fires were started at so many locations simultaneously they appear however to have helped prevent fires from spreading farther east into the main business and residential section of Nagasaki end of section three of the atomic bombings of Hiroshima and Nagasaki section four of the atomic bombings of Hiroshima and Nagasaki this is a Librebox recording all Librebox recordings are in the public domain for more information or to volunteer visit Librebox.org the atomic bombings of Hiroshima and Nagasaki by the Manhattan engineer district june 29th 1946 section four including total casualties and the nature of an atomic explosion total casualties there has been great difficulty in estimating the total casualties in the japanese cities as a result of the atomic bombing the extensive destruction of civil installations hospitals fire and police department and government agencies the state of utter confusion immediately following the explosion as well as the uncertainty regarding the actual population before the bombing contribute to the difficulty of making estimates of casualties the japanese periodic censuses are not complete finally the great fires that raged in each city totally consumed many bodies the number of total casualties has been estimated at various times since the bombings with wide discrepancies the Manhattan engineer district's best available figures are table a estimates of casualties pre raid population Hiroshima 255 000 Nagasaki 195 000 dead Hiroshima 66 000 Nagasaki 39 000 injured Hiroshima 69 000 Nagasaki 25 000 total casualties Hiroshima 135 000 Nagasaki 64 000 the relation of total casualties to distance from x the center of damage and point directly under the airburst explosion of the bomb is of great importance in evaluating the casualty producing effects of the bombs this relationship for the total population of Nagasaki is shown in the table below based on the first obtained casualty figures of the district table b relation of total casualties to distance from x distance from x in feet zero through 1640 killed 7505 injured 960 missing 1127 total casualties 9592 killed per square mile 24700 distance from x and feet 1640 through 3300 killed 3688 injured 1478 missing 1799 total casualties 6965 killed per square mile 4040 distance from x in feet 3300 through 4900 feet killed 8678 injured 17137 missing 3597 total casualties 29412 killed per square mile 5710 distance from x in feet 4900 through 6550 killed 221 injured 11958 missing 28 total casualties 12207 killed per square mile 125 distance from x in feet 6,550 through 9850 feet killed 112 injured 9,460 missing 17 total casualties 9,589 killed per square mile 20 no figure for total pre-raid population at these different distances were available such figures would be necessary in order to compute percent mortality a calculation made by the british mission to japan and based on a preliminary analysis of the study of the joint medical atomic bomb investigating commission gives the following calculated values for percent mortality at increasing distances from x table c percent mortality at various distances distance from x in feet 0 through 1000 feet percent mortality 93% distance from x 1000 through 2000 feet percent mortality 92% distance from x 2000 through 3000 feet percent mortality 86% distance from x 3000 through 4000 feet percent mortality 69% distance from x 4000 through 5000 feet percent mortality 49% distance from x 5000 through 6000 feet percent mortality 31.5% percent distance from x 6000 through 7000 feet percent mortality 12.5 percent distance from x 7000 through 8000 feet percent mortality 1.3 percent distance from x 8000 through 9000 feet percent mortality 0.5 percent distance from x 9000 through 10 000 feet percent mortality 0 it seems almost certain from the various reports that the greatest total number of deaths were those occurring immediately after the bombing the causes of many of the deaths can only be surmised and of course many persons near the center of explosion suffered fatal injuries from more than one of the bomb effects the proper order of importance for possible causes of death is burns mechanical injury and gamma radiation early estimates by the Japanese are shown in D below table D cause of immediate deaths city of Hiroshima cause of death burns 60% of total falling debris 30% of total other 10% of total city of Nagasaki burns 95% of total falling debris 9% of total flying glass 7% of total other 7% the nature of an atomic explosion the most striking difference between the explosion of an atomic bomb and that of an ordinary TNT bomb is of course in magnitude as the president announced after the Hiroshima attack the explosive energy of each of the atomic bombs was equivalent to about 20 000 tons of TNT but in addition to its vastly greater power an atomic explosion has several other very special characteristics ordinary explosion is a chemical reaction in which energy is released by the rearrangement of the atoms of the explosive material in an atomic explosion the identity of the atoms not simply their arrangement is changed a considerable fraction of the mass of the explosive charge which may be uranium 235 or plutonium is transformed into energy Einstein's equation E equals mc squared shows that matter that is transformed into energy may yield a total energy equivalent to the mass multiplied by the square of the velocity of light the significance of the equation is easily seen when one recalls that the velocity of light is 186 000 miles per second the energy released when a pound of TNT explodes would if converted entirely into heat raise the temperature of 36 pounds of water from freezing temperature 32 degrees Fahrenheit to boiling temperature 212 degrees Fahrenheit the nuclear fission of a pound of uranium would produce an equal temperature rise in over 20 million pounds of water the explosive effect of an ordinary material such as TNT is derived from the rapid conversion of solid TNT to gas which occupies initially the same volume as the solid it exerts intense pressures on the surrounding air and expands rapidly to a volume many times larger than the initial volume a wave of high pressure thus rapidly moves outward from the center of the explosion and is the major cause of damage from ordinary high explosives an atomic bomb also generates a wave of high pressure which is in fact of much higher pressure than that from ordinary explosions and this wave is again the major cause of damage to buildings and other structures it differs from the pressure wave of a blockbuster in the size of the area over which high pressures are generated it also differs in the duration of the pressure pulse at any given point the pressure from a blockbuster lasts for a few milliseconds a millisecond is one thousandths of a second only that from the atomic bomb for nearly a second and was felt by observers both in japan and in new mexico as a very strong wind going by the next greatest difference between the atomic bomb and the TNT explosion is the fact that the atomic bomb gives off greater amounts of radiation most of this radiation is light of some wavelength ranging from the so-called heat radiations of very long wavelength to the so-called gamma rays which have wavelengths even shorter than x-rays used in medicine all of these radiations travel at the same speed this the speed of light is 186 000 miles per second the radiations are intense enough to kill people within an appreciable distance from the explosion and are in fact the major cause of deaths and injuries apart from mechanical injuries the greatest number of radiation injuries was probably due to the ultraviolet rays which have a wavelength slightly shorter than visible light and which caused flash burn comparable to severe sunburn after these the gamma rays of ultra short wavelength are most important these cause injuries similar to those from overdoses of x-rays the origin of the gamma rays is different from that of the bulk of the radiation the latter is caused by the extremely high temperatures in the bomb in the same way as light is emitted from the hot surface of the sun or from the wires of an incandescent lamp the gamma rays on the other hand are emitted by the atomic nuclei themselves when they are transformed in the fission process the gamma rays are therefore specific to the atomic bomb and are completely absent in tnt explosions the light of longer wavelength visible and ultraviolet is also emitted by a tnt explosion but with much smaller intensity than by an atomic bomb which makes it insignificant as far as damage is concerned a large fraction of the gamma rays is emitted in the first few microseconds millions of a second of the atomic explosion together with neutrons which are also produced in the nuclear fission the neutrons have much less damage effect than the gamma rays because they have a smaller intensity and also because they are strongly absorbed in air and therefore can penetrate only two relatively small distances from the explosion at a thousand yards the neutron intensity is negligible after the nuclear emission strong gamma radiation continues to come from the exploded bomb this generates from the fission products and continues for about one minute until all of the explosion products have risen to such a height that the intensity received on the ground is negligible a large number of beta rays are also emitted during this time but they are unimportant because their range is not very great only a few feet the range of alpha particles from the unused active material and fissionable material of the bomb is even smaller apart from the gamma radiation ordinary light is emitted some of which is visible and some of which is the ultraviolet ray mainly responsible for flash burns the emission of light starts a few milliseconds after the nuclear explosion when the energy from the explosion reaches the air surrounding the bomb the observer sees then a ball of fire which rapidly grows in size during most of the early time the ball of fire extends as far as the wave of high pressure as the ball of fire grows its temperature and brightness decrease several milliseconds after the initiation of the explosion the brightness of the ball of fire goes through a minimum then it gets somewhat brighter and remains at the order of a few times the brightness of the sun for a period of 10 to 15 seconds for an observer at six miles distance most of the radiation is given off after this point of maximum brightness also after this maximum the pressure waves run ahead of the ball of fire the ball of fire rapidly expands from the size of the bomb to a radius of several hundred feet at one second after the explosion after this the most striking feature is the rise of the ball of fire at the rate of about 30 yards per second meanwhile it also continues to expand by mixing with the cooler air surrounding it at the end of the first minute the ball has expanded to a radius of several hundred yards and risen to a height of about one mile the shock wave has by now reached a radius of 15 miles and its pressure dropped to less than one tenth of a pound per square inch the ball now loses its brilliance and appears as a great cloud of smoke the pulverized material of the bomb this cloud continues to rise vertically and finally mushrooms out at an altitude of about 25,000 feet depending upon meteorological conditions the cloud reaches a maximum height of between 50,000 and 70,000 feet in a time of over 30 minutes it is of interest to note that Dr. Hans Bet then a member of the Manhattan engineer district on loan from Cornell University predicted the existence and characteristics of this ball of fire months before the first test was carried out to summarize radiation comes in two bursts an extremely intense one lasting only about three milliseconds and a less intense one of much longer duration lasting several seconds the second burst contains by far the larger fraction of the total light energy more than 90 percent but the first flash is especially large in ultraviolet radiation which is biologically more effective moreover because the heat in this flash comes in such a short time there is no time for any cooling to take place and the temperature of a person's skin can be raised 50 degrees centigrade by the flash of visible and ultraviolet rays in the first millisecond at a distance of 4,000 yards people may be injured by flash burns at even larger distances gamma radiation danger does not extend nearly so far and neutron radiation danger is still more limited the high skin temperatures result from the first flash of high intensity radiation and are probably as significant for injuries as the total dosages which come mainly from the second more sustained burst of radiation the combination of skin temperature increase plus large ultraviolet flux inside 4,000 yards is injurious in all cases to exposed personnel beyond this point there may be cases of injury depending upon the individual sensitivity the infrared dosage is probably less important because of its smaller intensity end of section four of the atomic bombings of Hiroshima and Nagasaki