336

Modern French Engine. [Book III,

The following figure of a modern French fire-engine is from the Ma-nuel du Fondeur ; Paris , 1829. It consists of two cylinders and an airvessel arranged in the usual way. One of the pumps, and half of the airchamber, is shown in section. The cistern is more elevated than in Eng-lish or American engines, and from the consequent height of the leverswould seem more inconvenient to be worked. The suction pipe is ofcopper with folding joints, and a perforated hollowball at the extremitytoprevent dirt or gravel from entering with the water. A short leatherntube connects this pipe with the suction cock. This engine is worked atthe ends of the carriage, and the piston rods are connected to the leverbyslings, and made to rise and fall in a perpendicular position by radius barsjointed to the upper ends of the latter, and to permanent pieces that pro-ject from the frame that Supports the fulcrum.

No. 152. Modern French Fire-engine.

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The elevation of the jet depends upon the pressure to which the air inthe air chamber is subjected; the elasticity or spring of that fluid beinginversely as the space it is made to occupy. Before an engine is set toWork the mterior of the chamber, like that of all erwpty vessels, (to use avulgär solecism,) is filled with common air, of that degree of density inwhich it appears near the earth’s surface ; but when the pumps are set towork, the water forced by them into the chamber crowds the air into thedome or upper part of that vessel, whence there is no passäge for its es-cape; and, as the liquid accumulates, the air is Condensed more and more,until, by its reaction on the surface of the water, it drives the latter throughthe jet or hose pipe, and with a force exactly proportioned to the degreeof its compressure. Thus if the volume of air in the chamber be com-pressed into half its bulk, the jet would rise to about 32 or 33 feet, (if notretarded by friction, angles or other imperfections in the pipe;) and if itwere made to occupy one third of its former space, its spring would bethree times greater than common air, and would force the jet to an eleva-tion of about 64 or 66 feet; and so on. A tabular Statement, similar to