-- .)

nq. Of thi Alir-Pump*

dible sioise: during which time, Me flafk will desceod, and pull downrthat end of the beam. When the noiseisover, put as many grains into-the scale at the otherend as will restore the equilibrium; and they willsliew exactly the weight of the quantity of air which has got into theflafk, and filled it. lf the flafk holds an exact quart, it will be found,,that 17 grains will restore the equipoife of the balance, when thequicksilver stands at 29s inches in the barometer : which fhews, thatwhen the air is at a mean rate of density, a quart of it weighs17 grains: it weighs more when the quicksilver stands higher ; andlefs when it stands lower.

2. Place the fmall receiver O (Fig. 1.) over the hole i in the:pump-plate, and upon exhausting the air, the receiver will be fixeddown to the piate by the pressure of the air on its outside, which islest to act alone, without any air in the receiver to act against it: andthis pressure will be equal to as many times 15 pounds, as there aresquare inches in that part of the piate which the receiver covers;which will hold down the receiver so fast, that it cannot be got off,until the air be let into it by turning the cock k ; and then itbecomes loofe.

Fig. 5. 3. Set the little glafs AB (which is open at both ends) over the

hole i upon the pump-plate LL, and put your hand dose upon thetop of it at B; then upon exhausting the air out of the glafs, youwill sind your hand prested down with a great weight upon it; sothat you can hardly release it, until the air be readmitted into theglast by turning the cock k ; which air, by acting as strongly upwardagainst the hand as the external air acted in prefling it downward, willrelease the hand from its consinement.

Fig. 6. 4. Having tied a pieee of wet bladder b over the open top of the

glast A (which is also open at bottom) fet it to dfy, and then thebladder will be tight like a drum. Then place the open end A uponthe pump-plate, over the hole r, and begin to exhaust the air out ofthe glast. As the air is exhausting, its spring in the glast will beweakened, and grve way to the pressure of the outward air on thebladder, which, as it is prested down, will put on a spherical concavefigure, which will grow deeper and deeper, until the strength of thebladder be overcome by the weight of the air; and then it will breakwith a report as loud as that of a gun.—If a flat pieee of glast be laidupon the open top of this receiver, and joined to it by a flat ring ofwet leather between them; upon pumping the air out of the receiver,the pressure of the outward air upon the statj glast will break it all topieces.

5. Immerso