TELEGRAPH. We may class these useful contrivances in two groups—the Mecha nical and the Electra -Mechanical Telegraphs :— Most of the older telegraphs consisted of boards or wooden arms, which according to the positions assumed, signified the letters of the alphabet. The kind generally used in France at the end of the last century was the T telegraph of M. Chappe. Four arms shaped something like a T were moved on three joints at the upper angles. M. Chappe communicated his intelligence letter by letter, and simplified the movements by using an alphabet of only sixteen letters. Such tele graphs were first erected on a line commencing at the Louvre, in Paris, and proceeding by Montmartre and other elevated points to Lisle, in order to communicate between the Committee of Public Safety and the combined armies in the low countries.
The advantages of such extraordinary cele rity of communication were so obvious that, in England and other countries, many plans were immediately brought forward, some of which differed materially from that which had been successfully put in practice in France. These plans are mostly separable into two classes : shutters which open or close certain apertures made to receive them ; or arms moveable on pivots. A shutter-apparatus, submitted to the Admiralty in 1795, by Lord George Murray, was adopted in the first government line of telegraphs established in England, in 1796, between London and Dover. It was employed by the Admiralty until 1816.
In 1816 it was determined to change the Admiralty telegraphs into semaphores con. strutted on the principle of those used in France, with the improvements suggested by Sir Home Popham. These semaphores had been introduced on the French coast in 1803. They consisted of upright posts with two or three moveable arms. Sir Home Popham's telegraph had two arms on one post ; but as they were mounted upon separate pivots, each of them could assume six different positions, and the two together were capable of affording 48 signals. This kind of telegraph continued to be used at the government stations till the introduction of the electric telegraph., Several modes of telegraphic communication without machinery, or with something which may be held in the hands, have been devised, especially for the purpose of directing military operations, or of conveying speedy intelligence in time of war, where no line of ordinary tele.
graphs can be established. One such method was by circular discs of wood held by men in particular positions ; another by a white handkerchief varied in position ; another by two small flags ; and another by stationing a few men in pre-arranged positions, &-c.
Marino telegraphic communication is an object of even greater importance than that on land, since there are many circumstances which render communication between vessels at sea impracticable by any other means than by 'signals, and that sometimes in cases of the greatest emergency. But, although naval signals have been necessarily long used, and flags of various forms and colours have been extensively employed for the purpose of making them, it was not till within a compa ratively recent period that they were reduced to anything like an efficient telegraphic system. Lord Howe proposed a system of numbering the flags need as signals, and also prepared a signal-book which was long used by the Admiralty. An improved code of signals, suggested by Sir Home Popham, was after wards adopted. A very useful system of flag telegraphing was introduced a few years ago by Mr. Watson, for the use of commercial shipping, and for maintaining communication between a vessel at sea and any of the stations on the coasts of the British islands, whence communication might be made by telegraph inland. • Electric Telegraphs.—It is chiefly to the' joint labours of Messrs. W. F. Cooke and Professor Wheatstone that electric telegraphs owe their practical application. Their tele graph, which was patented in 1837, acted upon principles founded on Oersted's celebrated discovery, that a magnetic or compass needle may, through the agency of a voltaic current, be invested with an artificial polarity. The first lino was laid down upon the London Rlackwall Railway ; the second from Londe)] to West Drayton ; and • the third (in 1840 from London to Gosport.