Page:The American Cyclopædia (1879) Volume XV.djvu/637

 TELEGRAPH 609 of the earliest of which is that of Dr. Robert Hooke described in the " Philosophical Trans- actions " for 1684. It consisted of 24 symbols formed of blocks of wood, representing alpha- betic characters, and six more formed of curved lines to be used as arbitrary signals. These were to be exposed in succession in an elevated frame at some conspicuous point, and, being observed at another station, were to be there" repeated and sent forward to the next, and so MI. At night torches or other lights were to substituted for the wooden figures. The irst working telegraph of much importance ras that known as Chappe's, invented in 1792, rhich was brought into use during the wars " the French revolution. At the top of a tall was attached a cross bar upon a pivot, so lat it could be easily turned from a horizontal an inclined position. Each end of this cross carried a short arm, which could also be irned upon its pivot so as to stand in any iition in relation to the bar. The movements re made by means of ropes which passed irough the bar and down the post. This ap- itus admitted of 256 distinct signals; but Chappe limited its use in great part to 16 lals, each one of which represented a let- jr of the abbreviated alphabet he had con- icted. Chappe's method has been gener- ly adopted, all the alleged improvements in being of minor importance. Mr. R. Lovell Igeworth about the same time brought be- )re the public his plan of a telegraph, or as he lied it telelograph or tellograph, by which e signals represented numbers, the meaning which would be found in the dictionary pre- ' for this system. The signals were made )y means of four pieces of wood, each one in form of a long isosceles triangle, placed ir together, each supported upon a pivot Hind which it could be turned in any direc- lon. The movements of each were limited to zero. The first triangle or pointer repre- ited units, the second tens, the third hun- reds, and the fourth thousands, so that any imber might be expressed that did not con- lin the figure 8 or 9. The admiralty tele- iph proposed by Lord G. Murray was used in England from 1795 to 1816, when it gave place to that known as the semaphore (Gr. cijua, a sign, and 0epe<v, to carry), which the French had adopted in 1803. This consisted of six conspicuous boards or shutters set in a frame, each of which could be turned upon its axis so as to present either its edge or its broad surface to the next station. The movements represented figures, and a series of numbers was indicated by their combinations. Some of these stood for the letters of the alphabet, and the others for arbitrary signals. The French semaphore (also known as signal posts) consisted of three or more arms attached by pivots to an upright post, admitting of motion in any direction, and indicating by their va- rious positions either figures or letters. Many modifications of this apparatus were used. For telegraphic communication at sea, flags of various colors have long been used. (See SIGNALS, NAVAL.) In 1835 Gauss proposed to employ a small heliotrope or mirror for re- flecting rays of light from the sun or an artifi- cial source as a means of communicating sig- nals. With a mirror so small that it may be carried in the waistcoat pocket, flashes of light may be clearly perceived for 12 m. or more, and, the mirror being gently moved on some established system, the appearance and disap- pearance of the flashes may indicate letters or words. By this device time can be saved, tel- escopes dispensed with, and the signals seen only by those for whom they are intended. Francis Galton, the African traveller, proposed a plan similar to this at a meeting of the roy- al geographical society, and described an opti- cal arrangement he had devised by which the operator may know if the mirror is directed aright. Among the later publications upon the telegraphs adopted previous to the electric telegraph, are papers in the "Journal of the Society of Arts," vols. xxvi., xxxiv., xxxv., and xxxvi. ; " A Treatise explanatory of a new Sys- tem of Naval, Military, and Political Telegraph- ic Communications," &c., by John Macdonald (London, 1817) ; "Description of the Universal Telegraph for Day and Night Signals," by C. W. Pasley (London, 1823) ; and Edgeworth's "Essay on the Art of conveying Secret and Swift Intelligence," in the "Transactions of the Royal Irish Academy," vol. vi. The ad- vantage of all these methods of telegraph- ing, which may be described in general as the optical method, is, that they employ nature's great highways, which cost nothing; the dis- advantages are, that the signals cannot record themselves, but require the constant atten- tion of an observer, and can be used only for moderate distances and in favorable weather. Moreover, the expense is great compared with the meagre intelligence which is communi* cated. The semaphore between London and Portsmouth, 72 m., which could be used less than one fifth of the time, required an annual expenditure of 3,403. ELECTEIC TELEGRAPH. The various kinds of electric telegraphs may be classified in two ways. In the first place, they differ in regard to the source from which the electricity is derived. In the present state of science, five independent sources of elec- tricity are recognized: 1, friction; 2, chemi- cal action ; 3, magnetic induction ; 4, heat ; 5, physiological actions. The difficulty of insu- lation unfits frictional electricity for this work, except at short distances and in dry air. The fourth and fifth sources must be rejected as insufficient for practical use. Successful tel- egraphs must rely on electricity produced by chemical action or magnetic induction. In the second place, electric telegraphs may be classified according to that one of the five special effects of electricity which is selected as the means of delivering the message when
 * ht, and indicated the first seven numerals