TELEGRAPH. The name given to a mechanical contrivance for the rapid com munication of intelligence by signals. Of late years, the term semaphore has been introduced by the French, and frequent ly adopted by English writers.
Although the art of conveying intelli gence by signals was practised in the ear liest ages, and is known even to the ru dest savages ; and although its impor tance is not only obvious, but continually felt wherever civilization is established, it has been allowed to remain in its orig inal state of imperfection down almost to our times. The first description of a tele graph universally applicable was given by Dr. Hooke. The method which he proposed,. for it was not carried into prac tice, consisted in preparing as many dif ferent shaped figures, formed of deal— for example, squares, triangles, circles, 8ze., as there are letters in the alphabet, and exhibiting them successively, in the required order, from behind a screen.
The first telegraph actually used was the invention of Chappe. It consisted of a beam which turned on a pivot in the top of an upright post, having a movable arm at each of its extremities ; and each different position in which the beam and its two arms could be placed at angles of 45° afforded a separate signal, which might represent a letter of the alphabet, or have any other signification that might be agreed upon. In 1803, the French erected semaphores along their whole line of coast, formed of an upright post, car rying two, or sometimes three beams of wood, each turning on its own pivot, one above the other. In 1807, Captain (now
General) Pasley published his Polygram mall° Telegraph, which was adopted in that year by the Admiralty instead of the shutter telegraph, and has continued in use ever since.
For day signals, the telegraph consists of an upright post of sufficient height, with two arms movable on the same pivot on the top of it, and a short arm, called the indicator, on one side ; as in the annexed figure. Each arm can hibit the seven tions, 1, 2, 3, 4, 5, 6, 7, besides the position called the stop, which points vertically wards, and is hid by the post. In order to show the number of nals that may be niade with this machine, we may suppose the arm nearest the indicator, reckoning in the order of the numbers as shown in the figure, to indicate tens, and the other units; then the signal re presented in the figure will be 17, which may be taken to denote a letter of the alphabet. If the arm on the left had the position indicated by 3, and that on the right the position indicated by 6, the sig nal would be 36. In this manner, the number of separate and independent sig nals, with their signification, will be as in the following table :— In 1803, Ronalds constructed a tele graph by galvanism, which worked through coils of 8 miles of wire, and Wedgewood, in 1817, formed and worked a voltaic telegraph.