post, the red glass is before the lamp and when the arm is let half way down the green glass comes in front of the lamp (fig. 13), thus the same action serves both for day and night signals. When the arm showing the red side projects in a horizontal direction, it indicates (in the day) “danger,” and so does the red light at night; when the arm is let down half way, it shows that caution is required, and the green glass then before the lamp shows the same signal at night; when the arm is let quite down out of sight, it shows safety, and so does the white light of the lamp thus freed from both screens of colored glass. Illustration: FIG. 14. (‡ ENGINE WHISTLE.) Each engine is provided with a whistle (fig. 14) blown by steam turned on from the boiler, which is used as a signal at any particular time, especially in tunnels or when there is a fog; there is also an arrangement by which each engine presses on a lever at the side of the tram as it passes, and causes a bell to ring at the station, to announce its approach, when about a quarter of a mile off. In some cases, as in foggy weather, when the usual signals cannot be seen, a packet of fulminating powder is placed on the rail, and this being exploded by the wheel of the engine as it passes over it, gives notice of its approach, &c. There are other signals, but these are the chief. ELECTRIC TELEGRAPHS. Illustration: (‡ ELECTRIC TELEGRAPH.) The power of transmitting messages to any distance or place to which a wire can be carried, and in a space of time too small to be reckoned, is without doubt one of the most wonderful inventions ever carried out by men’s hands. Although the signals are carried from place to place with a rapidity almost incredible, yet the electric fluid travels at a certain, although marvellously rapid rate. It is thought that light and the electric fluid both travel at the same rate, namely, 192,000 miles in a second, and if so, a message might be sent round the world (were it possible to carry on a wire) thrice in that small space of time. Illustration: FIG. 1. (‡ ACID BATTERY CELL.) Illustration: FIG. 2. (‡ BATTERY SERIES.) Illustration: FIG. 3. (‡ GROUNDING EFFECT.)