The immediate predecessor of the electric telegraph was the optical telegraph invented by Hooke, Newton’s assistant. The first occasion on which the optical telegraph was put to practical use was about the time of the French Revolution. In the year 1792 the whole of France was covered by a system of signals. The system consisted of a number of towers on the top of which “semaphores” were mounted which looked very much like the railway signals of the present day. By means of this system a message could be sent from Paris to the end of France in a few hours. When the war broke out in Austria in 1809, the great Napoleon profited by it to fall upon the Austrians unawares, giving them no time to make preparations to meet him. [Illustration: Fig. 52.--Optical Telegraph.] The Austrians were in alliance with the Bavarians, who suffered greatly by the war with Napoleon, and the Bavarian Government asked one of the professors at Munich to devise a system of telegraphy which should be superior to that used so successfully by Napoleon. The name of the professor was Soemmering. [Illustration: Fig. 53.--Soemmering’s First Electric Telegraph.] He started on the work at once, and four days after receiving the request he had devised the first electric telegraph which was capable of transmitting the letters _a_, _b_, _c_, _d_, _e_. That was all. The apparatus was quite simple and very ingenious. It consisted of one of Volta’s piles made of fifteen pairs of silver and iron plates separated by layers of felt soaked in salt water. This arrangement was the source of electricity. At the station to which the signals were to be sent there was a bottle or small tank of water with a large cork in the bottom of it. Through this cork five wires were drawn, and they led into water with which the bottle was filled. The five wires were made very long. They were covered with shellac varnish, and then they could be twisted together without being in actual contact. The strand of wire was stretched between the sending station and the receiving station. At the sending station the strand of wire was unravelled, and the five loose ends were marked with the letters _a_, _b_, _c_, _d_, _e_. The top of the Volta pile placed at the sending station gave positive electricity, and the bottom of the pile gave negative electricity. Now, suppose that the wire marked _a_ was brought into contact with the top of the pile, and the wire marked _b_ was brought into contact with the bottom of the pile. Then an electric current travelled all along the strand, through the pile and the two wires, and through the water between the two wires, thus making a complete circuit. The water in the bottle was then split up into hydrogen and oxygen, and bubbles of hydrogen appeared at the wire _b_ where it entered the water. The observer at the receiving station then understood that his companion at the sending station wanted to telegraph to him the letter _b_.