problem of transmitting, not signs, but the ordinary letters of the alphabet, so that the messages should be received in print, received the attention of the earliest telegraphic invent ors. The first sng-gestion of a type-printing apparatus originated as early as 1837 with Alfred Vail, an associate of Morse, but, believing that the method could not compete successfully with the ordinary Morse system of transmission, liis instrument was never perfected. Wheatstone next es sayed the solution of the problem, without reaching a successful issue,. although he constructed a working model which was operated before the Royal Polytechnic Institute in London in 1841.
House' s first type-printing instrument sufficiently practical to come into considerable use was that of House, an American electrician, who in 1847 successfully transmitted printed messages over an experimental line between Cincinnati and Jeffersonville, a distance of one liiindred and fifty miles. In 1849 this instrument, which was afterward greatly improved, was adopted cominercially on a line between New York and Philadelphia, and thereafter came into general use.
Hughes' s House apparatus has, however, been super seded by the type-printing- telegraph of Hughes, another American electri cian, which was brought out in 1833, and, as subsequently improved by his associate, Phelps, it has been adopted extensively throughout Europe and America.
Construction of mechanical construction of these devices is too complex to warrant a detailed description. It must suf fice to say that the type-printing instruments are operated by two methods. In one a type wheel is revolved by a step-by-step movement which is effected either by clockwork controlled by an electro-magnetic escapement or by the vibrating armature directly. In this class of type-writing- in struments the printing is done either by a clockwork mechanism or by the direct action of an electro-inagnet. To this class belong the instruments of Wheatstone, House, Breg,uet, Dujardin, and the stock- and market reporting instruments in general use. The " Universal Stock Printer " 051. 58,fig. 9) is a typical form of this telegraph. The other type-printing instruments have for their essential principle the synchronous movement of the transmitting and receiving apparatus at two or more stations, and the printing is effected without arresting the synchronous motion of the type wheel. The motion of the type wheel of one station and of the trans
mitting mechanism of the other is regulated by two separate sets of mechan ism, which keep exact time with each other. To this class belong- the instruments of Hughes, Farmer, Phelps, and others.
Transmission clever idea of reproducing at a distant station exact copies of writing, drawings, diagrams, charts, etc., telegraphically was first made practicable by Bakewell in 1847. Methods of fac-simile transmission have also been devised by Bain, Morse, Whitehouse, Gintl, Starer, Cowper, Robertson, Delany, and others.
The of Caselli, which belongs in this class, is exhibited partially in Figure 39 (pi. 57). Its operation will be understood from the following brief account condensed from Prescott. Two instruments simi lar to that shown in Figure 39 (fik 57) arc employed, one at the trans mitting and one at the receiving station. The actuating mechanism of this system is a heavy pendulum of iron swinging between two electro magnets which become alternately magnetic bv the action of a local bat tery, whose circuit is closed through these by the action of a second pendulum. The alternate action of the electro-magnets keeps the first pendulum in motion., and through this the remainder of the mechanism. At the right side of the frame are placed two covered metallic tablets, one of which, X, is shown in the Figure. Upon this is placed the original drawing, etc., to be transmitted at the sending station, or the prepared paper if at the receiving station, and, each instrument having two of these tablets, it is possible to transmit and receive a message at the same time. The two tablets are exactly alike, and it will be necessary in this account to refer to only one of them. Above the tablet X a frame, fi, q, is mounted upon .a vertical lever, A, 13, which turns npon its centre on a horizontal axis. The connecting-rocl, Z, is jointed to the lower end, B, of the lever, and to the rocl of the pendulum. Thus the swinging of the pendultim-rod causes the frame, fi, q, tO 1110VC to and fro over the convex surface of the tablet X. The adjustable counterpoise, K, K', serves to balance the weight of the frame, p, q, upon the centre of motion of the lever, A, 13, which is coincident with the axis of the cylinder of which X is a segment.