OF THE AIR-PUMP. 



211 



and set the spring c at bend upon the opposite ends of 

 the pins : then push down the slider d, and the spring 

 acting equally strong upon each mill, will set then both 

 a going with equal forces and velocities ; but the mill 

 a will run much longer than the mill 6, because the air 

 makes much less resistance against the edges of its 

 sails, than against the sides of the sails of b. 



Draw up the slider again, and set the spring upon the 

 pins as before ; then cover the machine with the receiver 

 J\l upon the pump plate, and having exhausted the 

 receiver of air, push down the wire P P (through the 

 collar of leathers in the neck q) upon the slider ; which 

 will disengage it from the pins, and allow the mills to turn 

 round by the impulse of the spring : and as there is no 

 air in the receiver to make any sensible resistance against 

 them, they will both move a considerable time longer than 

 they did in the open air ; and the moment that one stops, 

 the other will do so too. This shews that air resists 

 bodies in motion, and that equal bodies meet with different 

 degrees of resistance, according as they present greater 

 or less surface to the air in the planes of their motions. 



2. Take off the receiver M, and the mills ; 

 and having put the guinea a and feather 6 upon 

 the brass flap c, turn up the flap, and shut it 

 into the notch d. Then putting a wet leather 

 over the top of the tall receiver A B (it being 

 open both at top and bottom) cover it with the 

 plate C, from which the guinea and feather 

 tongs e d will then hang within the receiver. 

 This done, pump the air out of the receiver ; 

 and then draw up the wire/" a little, which by 

 a square piece on its lower end will open the 

 tongs e d ; and the flap falling down as at c, 

 the guinea and feather will descend with equal 

 velocities in the receiver ; and both will fall 

 upon the pump-plate at the same instant. 



N. B. In this experiment, the observers ought not to 

 P2 



LECT 

 VI. 



