To understand the manner of multiplying messages or signals on the same line, and using apparently the same electric current to perform different operations, the mind must revert to the theory already referred to, that a current of electricity does not consist of a stream of matter flowing like water through a conductor in one direction, but of particles of subtle ether, vibrating or oscillating in waves from and around the conductor which excites them; that the vibration of this line of waves proceeds at the rate of many thousand miles per second, almost with the velocity of waves of light, with which they are so closely related; that this wave current is susceptible of being varied in direction and in strength, according to the impulse given by the initial pressure of the transmitting and exciting instrument; and that some wave currents have power by reason of their form or strength to penetrate or pass others coming from an opposite direction. So that in the multiplex process, for instance, each transmission having a certain direction or strength and its own set of transmitting and receiving instruments, will have power to give its own peculiar and independent signal or message. Apparently there is but one continuous current, but in reality each transmission is separated from the others by an almost inconceivably short interval of time. Among the inventions in the class of Telegraphy should also be mentioned the dial and the printing systems. Ever since the electric telegraph was invented, attempts have been made to use the electric influence to operate either a pointer to point out the letters of the message sent on a dial, or to print them on a moving strip of paper; and also to automatically reproduce on paper the handwriting of the sender or writer of the message. The earliest efforts were by Cooke and Prof. Wheatstone of London, in 1836-37; but it was not until 1839, after Prof. Henry had succeeded in perfecting the electromagnet, that dial and printing telegraphs were successfully produced. Dial telegraphs consist of the combination with magnets, armatures and printed dial plate of a clock-work and a pointer, means to set the pointer at the communicating end (which in some instances has been a piano keyboard) to any letter, the current operating automatically to indicate the same letters at the receiving end. These instruments have been modified and improved by Brequet and Froment of France, Dr. Siemens and Kramer, and Siemens and Halske of Germany, Prof. Wheatstone of England, Chester and Hamblet of America, and others. They have been used extensively upon private and municipal lines both in Europe and the United States.