The electric telegraph may be said to have originated in a trivial incident. It occurred to professor Oersted, of Copenhagen, to try the effect of a galvanic current on the needle of the compass. He found it, on making the experiment, deflected, that is, turned aside from its usual bearing of due north and south. Professor Wheatstone applied this result very ingeniously. He arranged a series of needles, mounted like that of the compass, and found that he could turn any of these aside by galvanic currents, while the others remained at rest. It was evident, therefore, that if each needle were supposed to denote a letter, any letters might thus be indicated; and, consequently, if an arrangement of needles standing for so many letters, respectively, were placed at the distance of fifty or a hundred miles, and any of them were acted on by means of wires traversing the distance, a message could be despatched at one end of the line, and read off at the other from the deflected needles, by any person duly acquainted with the arrangement. A similar set of needles at the opposite end, would enable him, as certainly, to transmit a reply. The engraving represents the front of the telegraph, exhibiting the index, as it is denominated. The wires, which are suspended through the length of the line, are attached at either end to the telegraphic instruments, a branch wire being fastened to a large metallic surface, imbedded in the earth for completing the electric current. When at rest, the handles are down, and the pointers remain in their vertical position. The signals are given by two magnetic needles, or pointers, each suspended vertically on an axis passing through the dial, and, behind, another pointer is fixed on each corresponding axis. A portion of the conducting wire, many yards in length, is coiled round the galvanometer frame, in which the magnet moves, so as to subject the magnet to the multiplied deflecting force of the electric current. [Illustration]