179

Of the Air-Pump.

the earth to trembie all around to a considerabledistance.

From this experiment we have a very naturalaccount of the fires of mount Atttiu^ Vesuvius ,and other volcanos, they being probably setvr sire at first by the mixture of such metallinean d sulphureous particles.

The air pump being constructed the fame way piateXIV.a s the water-pump, whoever understands the Fig. 1 .one, vvill be at no loss to understand the other.

Having put a wet leather on the plate LLVheaif-the air-pump, place the glass receiveru P°n the leather, so that the hole i in the platejuay be within the glass. Then, turning thehandle F backward and forward, the air will bePumped out of the receiver; which will thenhe held down to the plate by the pressure of theexternal air, or atmosphere. For, as the handleF(Fig. 2 .) is turned backwards, it raises thepiston de in the barrel B K, by means of thewheel E and rack Dd: and, as the piston is lea-thered so tight as to fit the barrel exactly, noa ir can get between the piston and barrel andtherefore, all the air above d in the barrel ishfted up towards B , and a vacuum is made in. barrel from b to e-, upon which, part of theair in the receiver M (Fig. i.) by its spring,rulhes through the hole r, in the brass plateT, along the pipe G C G (which communi-cates with both barrels by the hollow trunk 1HK- an d pushing up the valve b, enters

2 ,° t ® e Vac ant place be of the barrel BK. For,erever the resistance or pressure is taken off,

~ air will ru n to that place, if it can find a^en, if the handle F be turned^ r ward, the piston d e will be depresied in thearre '» an d, as the air which had got into theN barrel