Os the Air-Pump .

ttpon the opposite ends of the pins: then pufh down the siider d, andthe spring acting 'equally strong oft each mill, will set them both agoing with equal forces and velocities: but the mill a will run muchlonger than the mill b, because the air makes much less resistance'against the edges of its falls» then against the sides of the falls of b.

Draw up the siider again, and set the spring upon the pins as before;then cover the machine with the receiver M upon the pump-plate, Fig. iand having exhausted the receiver of air, push down the wire P P(through the collar of leathers in the neck q) upon the siider; whichwill disengage it from the pins, and allow the mills to turn round bythe impulse of the spring and as there is no air in the receiver tomake any sensible resistance against them, they will both move a con-siderable time longer than they did in the open air; and the momentthat one stops, the other will do so too.—This shews that air resistsbodies in motionrand that equal bodies meet with different degreesof resistance, according as they prefent greater or less surfaces to theair, in the planes of their motions.

2. Take off the receiver M t and the mills; and having put theguinea a and feather b upon the brass flap c, turn up the flap, and stmtFig. 4it into the notch d. Then, putting a wet leather over the top of thetali receiver AB (it being open both at top and bottom) cover it withthe piate C, from which the guinea-and-feather tongs ed will thenhang within the receiver. This done, pump the air out of the re-ceiver ; and then draw up the wire f a little, which by a squarepiece on its lower end will open the tongs ed ; and the flap fallingdown, as at c, the guinea and feather will descend with equal velo-cities in the receiver; and both will fall upon the pump-plate at thefame instant. N. B. In this experiment, the observers ought not tolook at the top, but at the bottom of the receiver j in order to seethe guinea and feather fall upon the piate : otherwise, on accountof the quickness of their motion, they will efcape the sight of thebeholders.

II. To Jhcw the mceight of the air.

1. Having fitted a brass cap, with a valve tied over it to the mouthof a thin bottle or Florence flask, whose Contents are exäctly known,screw the neck of this cap into the hole i of the pump-plate : then,having exhausted the air out of the flask, and taken it off from thepump, let it be suspended ak one end of a balance, and nicely counter-poised by weights in the fcale at the other end : this done, raife upthe valve with a pin, and the air will rusli into the fl’lk with an au-

dible

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