 In 1849, a French physicist named Antonio-Louis Friseux did measure the speed of light. He repeated an unsuccessful experiment conducted by Galileo in the 1630s. Galileo's method was quite simple. He and an assistant each had lamps, which could be covered and uncovered at will. They climbed to the tops of hills around 1.5 kilometers apart. Galileo would uncover his lamp, and as soon as his assistant saw the light, he would uncover his. By measuring the elapsed time until Galileo saw his assistant's light, factoring in reaction times calculated earlier, and knowing how far apart the lamps were, Galileo reasoned he should be able to determine the speed of light. Given how fast light is, we know that the time interval Galileo was trying to measure was around 5 microseconds. The clocks available to him at that time could not measure that tiny a time interval. His conclusion was that light was very fast, if not instantaneous. As Galileo had done, Fitzhugh chose two high points, but in his case they were a good deal further apart at just over 8.5 kilometers. In place of covering and uncovering lanterns, he used shining light through the edge of a toothed wheel, whether the light beam got through the edge of the wheel depended on the wheel's position. If one of the gaps was in front of the light beam, it got through. If one of the teeth was in front of the light beam, it was blocked. To avoid the problem of human reaction time, Fitzhugh placed a mirror on the far hill instead of a person. He also added a partially reflected mirror to guide returning light to his eye. When Fitzhugh set the wheel spinning at slow speed, a flash of light that shot through one of the gaps would travel to the mirror on the distant hilltop, get reflected and travel back to Fitzhugh so fast that the gap was still in place. The wheel had not had time to move a tooth in the way of the beam of light to block its return. Fitzhugh then increased the speed of the wheel until the light moving through each gap of the mirror and back encountered a tooth instead of the gap on its return. This blocked the light from getting to his eye. Fitzhugh continued to make the wheel spin faster until eventually the light would shoot through a gap and by the time it traveled to the mirror and back, the tooth had moved completely across the line of sight, the beam of light returned just in time to move through the next gap and he could see it again. In super slow motion it would look like this. Dividing the number of teeth and the rotation rate, Fitzhugh could calculate the time it took for one tooth to move out of the way of the returning light. Dividing the distance by the time gave him the speed of light at 313 million meters per second. He was only off by four percent. Today we beam laser light through a vacuum and measure the timing with atomic clocks. Here is the current number. Wheel round to 300 million meters per second.