620 TELEGRAPH nometers in observatories are also made to run synchronously with a standard instrument by means of the electric current. Recently, the Harvard college observatory has established a telegraphic connection with Boston, and thence with the lines which diverge from that city, so that a uniform time can be distributed to all the railroad stations in New England. In a similar way Greenwich time is given to the whole of Great Britain. The application of the telegraph to the determination of longitudes has been described in the article COAST SUR- VEY, vol. iv., p. 759. Upon some railroads the telegraph is used with great advantage for regulating the running of trains. In numer- ous places telegraphs have been constructed for private purposes, and in London from the house of commons to the committee rooms. The transactions of the stock exchange in New York are telegraphed to the brokers' offices and the hotels, and are instantly and simul- taneously made known in a thousand different places, where they are sometimes recorded by automatic printing instruments. For this pur- pose a very rapid printer has been devised. The usual type and escape wheels are made very light, and are rotated, not by electricity, but by a spring. The current is reversed at every vibration, and the printing is effected by the power of a magnet, which is included in the same circuit with those that liberate the escape wheel; but it is made more slug- gish in action so that it does not perform its work until the arrest of the circuit wheel at a letter gives time for it to be charged. This instrument, which occupies only one sixth of a cubic foot of space, will print 800 let- ters a minute. A system of telegraphs for the use of large cities was devised by Wheat- stone, by which a company leases the use of a small wire by the year to individuals. For distances not exceeding 20 m. a copper wire no larger than a cotton thread is sufficient. Numbers of these, insulated by being wound with thread, may be brought together into one cord, and suspended from strong iron wires passed in different directions upon the houses. The latter, communicating with the ground at numerous points, will convey away all atmos- pheric discharges that might otherwise be troublesome. The "Law Telegraph Compa- ny" in the city of New York has established a complete system of communication by means of dial instruments between the leading law firms and the courts. A rapid system of sig- nalling is used, by which any member of the company can be put, through the agency of a central office, into direct private communica- tion with any other member, or with the courts of New York or Brooklyn. The Chester dial is employed by this company. In the automatic fire alarm, a circuit is closed by the expansion of metal under a rising temperature. The cir- cuit closer, which is called a thermostat, is attached to the ceilings of stores or dwellings, and is adjusted to work at a fixed tempera- ture. In the city of New York houses and stores furnished with these instruments are connected telegraphically with the fire patrol, the usual apparatus for indicating the locality of the fire being included in the system. The district telegraph system, which has been in- troduced in New York, Boston, and elsewhere, by which a messenger, policeman, or fireman can be summoned to any house that adopts it, is a still wider extension of Wheatstone's scheme. On a smaller scale, telegraphic com- munications may be kept up between the re- mote quarters of a ship or yacht; the elec- tro-magnetic bell-ringer may be used for do- mestic purposes, and the burglar alarm for the protection of private dwellings. By means of Batchelder's electro-magnetic tell-tale clock, the times are recorded when a watchman visits the different points of his beat. The most difficult piece of music may be punched out upon a moving strip of paper, and then played automatically by means of electro-magnetism. On the field of battle, telegraphic lines may be quickly extemporized, and an interchange of reports and orders may be maintained between the outposts of an army and headquarters. During the American civil war, telegraphic field trains were in use. A machine has been invented, operated by keys, which enables a reporter to secure a printed copy of the very words which come from the mouth of the ora- tor. In some countries, as in England, where the lines have been purchased by the govern- ment, the telegraphs are associated with the postal service. For short distances the pneu- matic telegraph is used, the written messages being driven through underground pipes by condensed air. For this purpose three engines of 50 horse power each are in constant ser- vice at the central post office in London. Multiple Telegraphy. During the last quar- ter of a century various attempts have been made to contrive ways by which two mes- sages should be sent at the same time, in the same or in opposite directions, over a single wire. Gintl, Edlung, Wartmann, Frischen, Sie- mens, Halske, Duncker, Starke, Rouvier, Zarite- deschi, Farmer, and Stearns have all experi- mented with this object, and some of them have invented ingenious instruments. In 1849 Sie- mens and Halske took out a patent in England for a method of transmitting simultaneously a plurality of messages. In 1855 Starke devised a method of sending two messages at the same time upon the same wire. By means of two keys, and two batteries of different intensi- ties, two independent receiving magnets wore worked at the other end of the line, either separately or together. In 1854 Siemens and Halske independently invented the differential method of sending two messages at the same time in opposite directions. About the same time Farmer devised a way of doing the same thing, using two auxiliary batteries in combi- nation with two principal batteries. The es- sential conditions for successful duplex tele-
