PNEUMATICS. 



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 ail 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. 



Join the two brass hemispherical cups, 

 A and B, together, (fig. 8) with a wet 

 leather between them, having a hole in 

 the middle of it ; then having screwed 

 off the handle at C, screw both the he- 

 mispheres put together into the pump- 

 plate, and turn the cock E, so that the 

 pipe may be open all the way into the 

 cavity of the hemispheres ; then exhaust 

 the air out of them, and turn the cock ; 

 unscrew the hemispheres from the pump, 

 and having put on the handle C, let two 

 strong men try to pull the hemispheres 

 asunder by the rings, which they will find 

 hard to do ; for if the diameter of the 

 hemispheres be four inches, they will be 

 pressed together by the external air with 

 a force equal to 190 pounds ; and to 

 show that it is the pressure of the air 

 that keeps them together, hang them by 

 either of the rings upon the hook of a 

 wire in the receiver of the air-pump, and, 

 upon exhausting the air out of the re- 

 ceiver, they will fall asunder of them- 

 selves. 



Set a square phial upon the pump-plate, 

 and having covered it with a wire cage, 

 put a close receiver over it, and exhaust 

 the air out of the receiver; in doing 1 

 which, the air will also make its way out 

 of the phial, through a small valve in its 

 neck. When the air is exhausted, turn 

 the cock below the plate to re-admit the 

 air into the receiver; and as it cannot 

 get into the phial again, because of the 

 valve, the phial will be broken into some 

 thousands of pieces by the pressure of the 

 air upon it. Had the phial been of a 

 round form, it would have sustained this 



