Now apply this principle to the harmonic telegraph and you can understand its operation. At the transmitting-end of a line of wire there are a certain number of forks or reeds kept vibrating continuously. These reeds each have a fixed rate of vibration and bear a harmonic relation to each other so as not to have sound-interference or "beats." At the receiving-end of the line there are as many electromagnets as there are transmitting-reeds, and each magnet has a reed or strap in front of it tuned to some one of the transmitting-reeds, so that each transmitting-reed has a mate in exact harmony with it at the receiving-end of the line. Keys are so arranged at the transmitting-end as to throw the tones corresponding to them to line when depressed. In other words, when the key belonging to battery B and vibrator 1 is depressed (see Fig. 4) the effect is to send electrical pulsations through the line corresponding in rate per second to that of the vibrator. The same is true of battery B' and vibrator 2. During the time any key is depressed--we will say of tone No. 1--this tone will be transmitted through the line and be reproduced by its mate--the one tuned in accord with it--at the receiving-station. By a succession of long and short tones representing the Morse code a message can be sent. Numbers two, three and four might be sending at the same time, but they would not interfere with number one or with each other. In 1876-7 the writer succeeded in sending eight simultaneous messages between New York and Philadelphia by the harmonic method. [Illustration: Fig. 4. In this diagram, 1 and 2 are tuned reeds; 1A 2A are receivers tuned to the reeds 1 and 2 respectively; 1 and 1A are in unison, also 2 and 2A, but the two groups (the 1s and the 2s) differ from each other in pitch.] There were two ways of reading by the harmonic method. One was by the long and short tone-sounds and the other by the ordinary sounder. The vibration of the receiving-reed was made to open and close a local circuit like a common Morse relay and thus operate the sounder. It is useless to try to send a message if the sender and receiver are out of tune with each other in this system. What is true in science is true in life. If we are out of tune with our surroundings we only beat the air, and our efforts are in vain. We get no sympathetic response. CHAPTER XIV. WAY DUPLEX SYSTEM.