The practical working of a telegraph involves such a multiplicity of I details, that we cannot attempt in an article of this kind to convey much minute information. Still, however, there are certain prominent I features which a reader moderately acquainted with electrical science will readily understand. An electric telegraph, whether for land or sea, consists of three essential parts : 1st, the transmitting apparatus, for generating electric action at one end, known as the snider; an electrode or insulated path along which the electricity may travel, and familiarly known as the line or the wire, or, in the submarine tele graph, the cable ; 3rdly, the apparatus used at the other end of the line to render evident the signals forwarded by the transmitting appa ratus: this is called the rccciring apparatus or instrument, or simply the instrument.
And first as to the transmitting apparatus. This is usually a voltaic battery, consisting of alternate pairs of copper and amalgamated zinc plates, arranged in troughs of some compact wood made tight with marine glue, subdivided into compartments by means of slate, or the troughs are of glazed gutta percha. Each compartment contains the copper of one pair and the zinc of the next. [GALVANIC BATTERY.] After the !late/ have been introduceal, the compartments are filled with sand, which is moistened with dilute sulphurie acid (I stroug acid to 15 water). This arrangement diminishes the risk of leakage, and lessens the amount of evaporation. The acid has to be renewed once In ten or fourteen days, according to the amount of work performed by the telegraph. In the Electric Telegraph Company'a central office at Lothbury the batteries are contained in two long narrow chambers in the basement of the building. There are upwards of sixty Daniell's batteries at work : they take rank as sixes, twelves, and twenty-fours, according to the number of their elements or elates, the twenty-fours working the longer lines, and the smaller batteries the shorter circuits. A twenty-four, when in full work, requires only a gill of dilute sul phuric acid per month, and its tine plates last three months. Other batteries require 1 lb. of sulphate of copper per month, with a little eulplutto of zinc or salt and water. The entire amount of electric power employed by this company throughout the country is said, on the authority of en article iu the ' Quarterly Review' (1854), to be pro ducal by S000 twelve-plate batteries, or 96,000 cells, which are hued with 1,500,000 square inches of copper, and about the same of tine. To work these batteries, 6 tons of acid and 55 tons of sand are required every year. When a distant station-clerk finds by the weak action of his needles that the battery is not up to its work, he sends word that it requires H refreshrncut," and it is accordingly served with its gill of sulphuric acid.
Another farm of battery which continues in action for a consider able time consists of plates of amalgamated zinc and gas coke excited by solid sulphate of mercury moistened with water ; they are arranged in compartments as in the sand battery.
Some lime are worked by means of the magneto-electrie machine.
The Atlantic cable during a portion of its brief existence was worked by a secondary current. An electric cable sunk in water is apt to become charged with electricity after the manner of a Leyden jar, and so to resist the passage of the current along its central conducting wire. The idea was, that by using a secondary current it would by its pulsations displace the charge, and allow the current to be transmitted. 3Ir. Whitehouse fed his double induction coils by means of what he calla a "perpetual maintenance battery." This battery consists of large plates of platinized silver, and amalgamated zinc, mounted in cells of gutta percha. There are several plates, both of silver and zinc, in each cell ; but all the zinc plates rest upon a longitudinal bar of metal at the bottom of the cell, and all the silver plates hang upon a similar bar at the top of the cell, so that thus there is virtually but a single stretch of silver, and a single stretch of zinc in operation. This arrangement is made because it enables any portion of either silver or rive to be removed for repair or renewal without stopping for a moment the operation of the battery. As any one lamina becomes imperfect, it can be taken out from its groove, and replaced. Each cell contains two thousand square inches of acting surface, and is charged with the usual mixture of acid and water, and there are ten such celLs in the battery. This combination is so powerful that when the broad strips of copper plate which form the polar extensions are brought into contact or separated, brilliant flashes are produced, accompanied by a loud crackling sound. The points of large pliers are made red.hot in five seconds when placed between them, and iron screws burn with vivid scintillation. These brilliant effects are, how ever, produced at the expense of the apparatus; the metallic surfaces from which they are emitted rapidly burning away during their con tinuance. In order to alleviate this injurious effect, contact is made and broken, during the transmission of electrical signals, by means of a key presenting a very large surface of metal. A horizontal bar, flattened at the top, turns backwards and forwards eivot-ways, and tilts its edges against twenty flat braes springs resembling in form the keys of a piano-forte, ten being on each side. A constant alight leak of the current is also continuously maintained through a coil of platinum wire placed in water. By this contrivance the injurious force of the spark is pretty well absorbed and destroyed. The cost of main taining thin magnificent battery at work is said not to exceed a shilling per hour.