Fire and garden engines are only applications of the pump to different purposes. The fire-engine has generally two cylinders and pistons, and has moreover an air-chamber for the purpose of making the stream of water continuous. It acts in this way:--The water is forced by the power of those who are pumping the engine into a vessel air-tight and full of air, having an opening which joins the “hose” at its lower part; the result is, that as the water is forced in faster than it can well escape, the air above it--becoming greatly compressed, and by its expansion between each stroke of the pistons--forces the water out, and so continues the stream or jet. Fig. 3 shows this air-chamber; _a_ joins to the hose _c_, and _b_ is in union with the forcing-pumps of the engine. The air is represented as it would be compressed to about half its bulk, for it at first filled all the air-chamber down to the openings. VALVES. Illustration: FIG. 1. (‡ FLAPPER VALVE.) Illustration: FIG. 2. (‡ BALL VALVE.) Illustration: FIG. 3. (‡ PLUG VALVE.) Illustration: FIG. 4. (‡ CLOSED PISTON VALVE.) Illustration: FIG. 5. (‡ OPEN PISTON VALVE.) Valves are contrivances to admit the passage of fluids or gases by their own pressure in one direction, and in such a manner that the same pressure shall of itself prevent their return or passage in the opposite direction, as in fig. 1, which is the piston of a common pump. There are almost innumerable varieties of valves, one consists of a ball of metal fitting into a cup which has a hole at the bottom (fig. 2). Another (fig. 3), is a plug of a conical shape fitting in the same way, and having a rod affixed to the top which passes through a hole in a piece of metal so as to guide it in its ascent and descent; this is the kind of valve used as a “safety-valve” in steam boilers, but having the pressure regulated by a spring or weights. Figs. 4 and 5 are representations of a kind of valve which forms the piston itself, and is very useful as a piston for a square wooden tube for temporary purposes, as on board ship, where any number may be fitted up at little trouble, time, or expense. There are many other valves besides these, as the sliding valves of steam-engines, &c. WHEELS. Illustration: FIG. 1. (‡ FLY-WHEEL.) Illustration: FIG. 2. (‡ POWER GEARS.) Illustration: FIG. 3. (‡ BEVELED GEARS.) Illustration: FIG. 4. (‡ SIDE-COGGED GEAR.) Illustration: FIG. 5. (‡ RATCHET GEAR.) Illustration: FIG. 6. (‡ CAPSTAN.) Illustration: FIG. 7. (‡ PULLEYS.) Illustration: FIG. 8. (‡ ENDLESS BAND PULLEYS.)