The elastic power of the air is always equivalent to the force which compresses it ; for if it were less, it would yield to the pressure, and be more compressed; were it greater, it would not be so much reduced ; for action and re-action are al ways equal, so that the elastic force of any small portion of the air we breathe, is equal to the weight of the incumbent part of the atmosphere ; that weight be ing the force which confines it to the di mensions it possesses.
To prove this by an experiment, pour some quicksilver into the small bottle, A, (fig. 7), and screw the brass collar, C, of the tube, B C, into the brass neck of the bottle, and the lower end of the tube will be immersed into the quicksilver, so that the air above the quicksilver in the bottle will be confined there. This tube is open at top, and is covered by the rscever, G, and large tube, EF ; which tube is fixed by brass collars to the receiver, and is closed at top. This preparation being made, exhaust the air out of the receiver, G, and its tube, by putting it upon the plate of the air pump, and the air will, by the same means, be exhausted out of the inner tube, B C, through its open top at C. As the receiver and tubes are ex hausting, the air that is confined in the glass bottle, A, will press so by its spring, as to raise the quicksilver in the inner tube to the same height as it stands in the barometer There is a little machine, consisting of two vanes of equal weights, independ ent of each other, and turn equally free on their axles in the frame. Each Vane has four thin arms or sails fixed into the axis : those of the one have their planes at right angles to its axis, and those of the other have their planes parallel to it. Therefore, as the former turns round in common air, it is but little resisted there by, because its sails cut the air with their thin edges ; but, the latter is much resist ed, because the broad side of its sails move against the air when it turns round. In each axle is a fine pin near the middle of the frame, which goes quite through the axle, and stands out a little on each side of it : under these pins a slider may be made to bear, and so hinder the vanes from going, when a strong spring is set or bent against the opposite ends of the pins.
Having set this machine upon the pump-plate, draw up a slider, and set the spring at bend on the opposite ends of the pins : then push down the slider, and the spring, acting equally strong upon each mill, will set them both a-go ing with equal forces and velocities ; but the first will run much longer than the last, because the air makes much less re sistance against the edges of its sails than against the sides of the other.
Draw up the slider again, and set the spring upon the pins as before ; then cover the machine with the receiver upon the pump-plate ; and having exhausted the receiver of air, push down the wire (through the collar of leathers in the neck) upon the slider; which will dis engage it from the pins, and allow the vanes to turn round by the impulse of the spring : and as there is no air in the re ceiver to make any sensible resistance against them, they will both move a con siderable time longer than they did in the open air; and the moment that one stops, the other will do so too. This shows that air resists bodies in motion, and that equal bodies meet with different degrees of resistance, according as they present greater or less surfaces to the air.
Take a tall receiver, covered at top by a brass plate, through which works a rod in a collar of leathers, and to the bottom of which there is a particular contrivance for supporting a guinea and a feather, and for letting them drop at the same instant. If they are let fall while the receiver is full of air, the gui nea will fall much quicker than the fea ther; but if the receiver be first exhaust ed, it will be found that they both ar. rive at the bottom at the same instant, which proves that all bodies would fall to the ground with the same velocity, if it were not for the resistance of the air, which impedes most the motion of those bodies that have the least momentum. In this experiment the observers ought not to look at the top, but at the bottom of the receiver, otherwise, on account of the quickness of their motion, they will not be able to see whether the guinea and feather fall at the same instant.
Take a receiver, having a brass cap fitted to the top with a hole in it ; fit one end of a dry hazel branch, about an inch long, tight into the hole, and the other end tight into a hole quite through the bottom of a small wooden cup ; then pour some quicksilver into the cup, and ex haust the receiver of air, and the pres sure of the outward air on the surface of the quicksilver will force it through the pores of the hazel, from whence it will descend in a beautiful shower, into a glass cup placed under the receiver to catch it.