htsitbaors.—An important factor in the serviceableness of a telegraph line is the insulation of the conductors from the posts. The wires are therefore supported by insulators of various forms, constructed usually of porcelain or glass. In Chauvin's insulator (fil. 57,17gs. 45, 46), an inner bell, which forms the insulator proper, is kept dry by the outer one. The wire is laid in the notch, a, and is held in position by a thin wire wound round the groove in the neck. Figure 44 shows another form, composed of an iron shield in which is cemented a porcelain capsule, forming the in sulator proper, from which depends a hook to which the line wire is attached. In the United States the use of glass insulators still prevails to a considerable extent, but they are inferior to porcelain. The Brooks in sulator, a much superior instrument, has, however, come into very general use. It is formed of an inverted cylindrical cup of porcelain. The iron hook for attaching the wire depends from its open mouth and is held in place in the cup by a plug of sulphur, which is poured in while melted. The porcelain cup is guarded by an outer shell or casing of iron of similar form. This iron casing, to which the cup is secured by cement, is furnished with a spike or screw, by which it may be attached to the arms of the post. The surface of the cement within and without the porcelain cup is saturated with paraffin. As this substance is repellent of moisture, a continuous film of the latter will never form over the surface of this insulator. It gives very satisfactory results on this account.
Una'ergrouna' it'ires.—Greater security against interruption of commu nication by storms, snow, ice, and malicious interference, but at increased cost of construction, is acquired by placing the wires underground. This practice obtains in most European cities, and largely throughout Germany, where extensive land lines connecting the chief cities and fortresses of the empire have been in operation with entire satisfaction for the past ten years. Such lines, when properly laid, are maintained at a comparatively trifling cost. This reform is now only beginning to be adopted in the United States, and the streets of American cities are still obstructed by overhead wires and disfigured by unsightly posts, which practically have long been banished from European cities.
First first trials with underground tele graph lines were made in 1846 by the Prussian lieutenant W. Siemens, but these were far from being successful, because of the imperfection of the insulation employed. The difficulty, however, was afterward completely overcome when attention was directed to the production of insulated sub marine cables, and the first successful underground line of any considerable length was laid in 18,-I between London and Liverpool.
Submarine first experiments to establish telegraphic communication under water were made in 1839, but were unsuccessful, owing to the defective nature of the insulation. This proved to be the
prime difficulty until gutta-percha was used. The first successful sub aqueous cable was laid across the Hudson River, between New York and Jersey City, in June, 184S, by the Magnetic Telegraph Company. It was followed in 1S49 by one laid in the harbor of Folkestone by \\Talker, whose line extended from a steamship to the shore, a distance of 3600 feet. The first commercially successful cable of considerable length was that laid in the English Channel in 1851 between Dover and Calais. At the present time submarine cables have been laid in all parts of the world. Of these only the Transatlantic cables will be noticed.
Transatlantic idea of establishing telegraphic communi cation between Europe and America originated with Cyrus W. Field of New York, who in 1856 succeeded in forming a company to carry the pro ject into execution. The first attempt, in 1S57, was a failure, owing to the breaking of the cable 274 miles from the Irish coast while being laid. In a second attempt, in IS58, the cable was actually laid, but it remained operative only long- enough to permit of the transmission of a few de spatches of congratulation, when it refused to act, owing, as was subse quently ascertained, to defective insulation. After the failure of the third attempt, in 1865, by the breakage of the cable after 1213 miles had been laid, the efforts to establish telegraphic intercourse between the two con tinents was at length crowned with success in July, 1866. The broken cable of the previous year was also found again, and united with the Amer ican continent. Since that time submarine lines have multiplied, and are now in successful operation in all quarters of the world. As yet no cable spanning the Pacific Ocean unites the western shore of America i.vith China or Japan, but the completion of the girdle about the entire globe will probably not long be delayed. There are now nine submarine North Atlantic cables between Europe and America, of which four belong to the Anglo-American Telegraph Company (laid respectively in 1869, 1873, 1874, and ISSo), two to the Western Union Telegraph Company (ISSI and IS82), one to the Direct United States Company (1875), one to the Compagnie Francaise (1879), and the I3ennett-Mackey Cable (1884).
Laying of Submarine the purpose of laying the oceanic cables, the latter are fornied into suitable lengths and stowed carefully in great tanks prepared to receive them in the holds of steamships built spe cially for this service, and supplied with self-releasing paying-out appli ances so devised that the cable shall not be subjected to injurious strain. Two vessels, for example, proceed to the task in mid-ocean, going in oppo site directions and paying out the cable as they proceed, until the terminal connections at the shore ends are safely made.