 Transmitting a message by visual signals is much faster than trying to deliver a physical message like a scroll of paper. And to remember why, recall that every message system has multiple delays within. So if we wanted to transmit a message by, let's say, a smoke signal here, there are a couple of different components that we need to take into account. First, we would have to light a fire. Then the message would have to travel between the two points. And then finally, we have to detect that a message is being transmitted. So let's start from the beginning. First, we have to light the fire and that is our transmission delay. So that happens right there. Next, we have the propagation delay, which is the time it takes to travel between these two points. So that's B right here. And we know that propagation delay is a function of two variables. It is a function of distance, which is how far our message needs to travel. And it is also a function of speed or how fast we can deliver the message. And the difference here is, of course, we know that, for example, a horse can deliver the message in with the speed rather of about 24 meters per second. But when we use the visual signal, we are actually traveling with the speed of light. And the speed of light is approximately 300 million meters per second. So much, much faster. And finally, we have our processing delay, which is C right here, which is the time it will take us to detect and verify that the message is being transmitted. And one important takeaway here is that the propagation delay is almost negligible for visual signals because the speed of light is so fast. So really, we are bound by the speed of the processing and transmission delays, which is to say that the slowest parts are actually our operators on both ends because they need to light a fire and interpret the signal. However, there's another and an even larger problem than the speed of our operators here. And that is using smoke signals doesn't actually give us a lot of flexibility in what kinds of messages we can transfer. In fact, we can only transfer one bit of information because either the fire is on or it's off. And if the other end knows what that means, that's great. But we're definitely not transmitting sentences, words, or even characters. So to solve this problem in around 1700s or mid 1700s, we invented the visual telegraph, which is also commonly known as a semaphore. And the idea for the semaphore was to encode visual signals by using these mechanical arms and different positions would mean different letters. So for example, if we grab the one we saw here, let's call that letter A. But we could come up with a different position, perhaps we could reverse it and that would give us letter B. And of course, we could repeat the same exercise here for all of the other letters in the alphabet and then we could transfer entire messages. And the first practical use of the visual telegraph was in 1794 during a war between France and Austria. And messages were transmitted between Paris and another French city called Lille. And there were actually 15 stations of these visual stations which transmitted the message over a total distance of about 230 kilometers. And the interesting part is that this entire distance was covered in about 9 minutes for each character. Which tells us a couple of really interesting things. So first of all, we know that we have 15 stations and it took us a total of 9 minutes. So if we do some math here and we know that it's 9 minutes divided by 15 stations, this works out to be approximately 36 seconds per station. So that is our processing and transmission delays because propagation delays can be very, very small. Or said differently, this is our A plus C. And this time is the total time at each station for each character. So this is another important distinction for the visual telegraph. Unlike, let's say, a horse where the entire message would arrive at once, we are delivering this message character by character. So if we send an A, that alone will take us 9 minutes. And then we would have to send another character and so on. So one important thing that happened once the visual telegraph or the semaphore came along is that all of a sudden we started seeing code books, which is to say we started encoding messages with fewer characters. Because the limitation for the visual telegraph was not the weight of the message, which is definitely the case for something like a carrier pigeon, but the length of the message became very, very important. And because the visual telegraph delivered messages so much faster because of the speed of light, it actually became very, very popular in Europe in the 1800s. But there are also a couple of problems with it. First of all, it is actually very expensive to operate because you need many different stations along the way. So we have 15 stations here. And each station would need people to operate. So in fact, because of this, it was effectively limited to military and government use. And the second problem is, of course, line of sight. We would need to build these stations where we would need to be able to see them and see them well and hopefully at long distances. And then the last one, which is an obvious one, is if the weather is poor or, for example, at night, we can't actually use the system. And so because of these problems, the visual telegraph actually got replaced as new and better methods came along later. But having said that, believe it or not, we are still using visual telegraph in some instances. So for example, we have flag semaphores, which are used on ships. This guy right here indicates letter A. We have letter B and letter C. And in fact, we have entire alphabet. So you can search for flag semaphore and find out how to communicate using these flag signals.