I'm a historian. I've struck on a curiosity about telecommunication technology in pre-modern times. I've read about a number of methods, and I'd like to ask some physics questions that got beyond my ken. This information is for my curiosity and creativity, not for any rigorous study, so I may be mentioning Wikipedia. Thank you for humoring me, and I hope I'm not asking too many questions :) I've now separated them into multiple threads by subject; this one ought to be about water. To conform with PF custom, I'll be adding the other questions to the threads after the first ones have been answered. Let anyone feel free to answer only those questions he prefers to. Water An idea I had myself which may or may not be sensible is burying a small, very long line pipe, and filling it with water. Looking into this, I read about a British "hydraulic telegraph," but its not quite the same since it's trying to relay more information with devices that can affect the water pressure more precisely than interests me. Obviously leaks, earthquakes, freezing, expense, and thirsty creatures are possible problems of this approach, but in theory... Q1. Suppose, as a signal, the sentry at one end of the pipeline pulls his plug. If the pipe was full, won't the fact of the drainage be communicated down the whole line fairly quickly?