Of the Air-Pump. l 3 i

which press down the top piece E upon the bar-rels : and the hollow trunk H (in Fig. 2.) iscovered by a box, as G H in Fig. i.

There is a glass tube Immmn open at bothends, and about 34 inches long; the upper endCommunicating with the hole in the purrip-plate,sod the lower end immersed in quicksilver at nm the vessel N. To this tube is fitted a woodentuler m m, called th egage, which is divided intoinches and parts of an inch, from the bottom at* (where it is even with the surface of the quick-silver) and continued up to the top, a littlebelow /, to 30 or 31 inches.

■ As the air is pumped outof the receiver M ,itislikewise pumped out of the glass tube lmn y be-cause that tube opens into the receiver throughthe pump-plate; and as the tube is graduallyEmptied of air, the qucksilver in the vessel N isforced up into the tube by the pressure of theatmosphere. And if the receiver could be per-fectly exhausted of air, the quicksilver wouldstand as high in the tube as it does at that timein the barometer: for it is supported by the[ame power or weight of the atmosphere in

both.

The quantity of air exhausted out of the re-ceiver on each turn of the handle, is always pro-portionable to the ascent of the quicksilver onl hat turn ; and the quantity of air remaining in'•he receiver, is proportionable to the defect ofthe height of the quicksilver in the gage, fromwhat it is at that time in the barometer.

1 shall now give an account of the experimentsfnade with the air-pump in my lectures ; shew- .n?g the resistance, weight, and elasticity of the

N 2 I. To