ELASTICITY OF AIR. 



55 



would follow that all bodies, whatever be their masses, should fall at the same 

 rate. Now the instances which most commonly come under our observation 

 seem to contradict this inference, for we find a piece of metal and a piece 

 of paper fall at very different rates, and still more different is the rate at 

 which a piece of metal and a feather would fall. The cause of this circum- 

 stance, however, is easily explained. The resistance offered by the air is 

 proportional to the quantity of surface which the body presents in the direction 

 of its motion. Now the metal may present a considerably less surface than 

 the feather, while the force which it exerts to overcome the resistance is many 

 times greater, because of its greater weight. Hence, it follows that the resist- 

 ance of the air produces a different effect on the metal compared with the 

 effect which it produces on the feather ; but all doubt will be removed if the 

 feather and the metal are allowed to fall in a chamber from which the air has 

 been withdrawn. A glass receiver is represented in fig. 10, which may be 



Fig. 10. 



placed on the plate of an air-pump, and on the top is placed a brass cover, 

 which is air-tight. Under this several brass stages are attached, constructed 

 in the manner of trap-doors on the hinges, and supported by small pins, which 

 project from the sides of a metal rod, passing through an air-tight collar in the 

 brass cover. By turning this metal rod, the pins may be removed from under 

 the trap-doors, and they will fall, disengaging whatever maj - be placed upon 

 them. Suppose a piece of coin and a feather be placed upon one of these 

 stages, supported by a projecting pin. This arrangement being made, let the 

 brass cover be placed on the receiver, so as to be air-tight, and let the receiver 

 be then exhausted by the pump. When a high degree of rarefaction has been 

 produced, let the rod be turned by the handle at the top, so as to remove the 

 pin from under the stage ; the coin and the feather will be immediately let fall, 

 and it will be observed that they will both descend at exactly the same rate, 

 and strike the bottom at the same instant. This is the experiment commonly 

 known as " the guinea and feather experiment." 



The surgical process called cupping, consists in removing the atmospheric 

 pressure from the part of the body submitted to the operation. A vessel with 



