PNEUMATICS. 139 



they did in the open air; and the moment that 

 one stops, the other will do the same. 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. 



2. Take a tall receiver A (Fig. 7.), 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 re- 

 ceiver is full of air, the guinea will fall much 

 quicker than the feather; but if the receiver be 

 first exhausted, it will be found that they both 

 arrive at the bottom at the same instant, whicli 

 proves that all bodies would fall to the ground with 

 the same velocity, if it were not for the resistance 

 of the air, w r hich impedes most the motion of those 

 bodies that have the least momentum. In this ex- 

 periment 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. 



3. 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 exhaust 

 the receiver of air ; the pressure 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. 



