184

Of the Air-Putftf,.

II. To shew the weight of the air..

1. Having fitted a brass cap, with a valvetied over it, to the mouth of a thin bottle orFlorence flask, whose contents are exactly known,,screw the neck of this cap into the hole i of thepump-plate : then, having exhausted the air outof the flask, and taken it off from the pump, letit be suspended at one end of a balance, and nicelycounterpoised by weights in the scale at the otherend : this done, raise up the valve with a pin, andthe air will rush into the flask with an audiblenoise: during which time, the flask will descend,and pull down that end of the beam. When thenoise is over, put as many grains into the scale atthe other end as will restore the equilibrium )and they will shew exactly the weight of thequantity of air which has got into the flask, andfilled it. If the flask holds an exact quart, itwill be found, that 16 grains will restore theequipoise of the balance, when the quicksilverstands at inches in the barometer: whichshews, that when the air is at a mean rate of den-stty, a quart of it weighs 16 grains : it weighssnore when the quicksilver stands higher; andless when it stands lower.

2. Place the small receiver 0 (Fig. 1.) over thehole i in the pump-plate, and upon exhaustingthe air, the receiver will be fixed down to theplate by the pressure of the air on its outside?which is left to act alone, without any air in thereceiver to act against it : and this pressure willbe equal to as many times 15 pounds, as thereare square inches in that part of the plate whi c ^the receiver covers ; which will hold down thereceiver so fast, that it cannot be got off, un«