I once watched a wonderful documentary about the history of long distance communication but have since forgotten its title, where I saw it, and how to find it again.
I really only remember two pieces of the documentary, which is why I want to find it and watch it again!
The first thing I remember is that the documentary began by discussing communication via a clepsydra (for which I found the following description from here).
Another queer signaling device, built and operated upon a novel principle, was an even greater wonder among the early peoples. This was known as a clepsydra. Fancy a tall glass tube with an opening at the bottom in which a sort of faucet was fixed. At varying heights sentences were inscribed about the tube. The tube, being filled with water, with, a float at the top, all was ready for signaling any of the messages inscribed on the tube to a station within sight and similarly equipped. The other station could be located as far away as a light could be seen. The station desiring to send a message to another exhibited its light. When the receiving station showed its light in answer, the tap was opened at the bottom of the tube in each station. When the float dropped until it was opposite the sentence which it was desired to transmit, the sending station withdrew its light and closed the tap. This was a signal for the receiving station to stop the flow of water from its tube. As the tubes were just alike, and the water had flowed out during the same period at equal speed, the float at the receiving station then rested opposite the message to be conveyed.
The other thing that I remember from the documentary was presented somewhat later. It was about the telegraph and the laying of the first transatlantic line. If I remember correctly, it said the first cable broke while being laid, but the laying of the second cable was successful (the first user being the monarch of England and, probably, the U.S. president). Later, this cable stopped working and another one was laid.
It would be great if someone knows what documentary I am talking about so I can both watch it again and share it with others.