PNEUMATICS. 



pressure, like an arch, without breaking ; 

 but as its Miles are flat, it cannot. 



Let a large piece of cork be suspended 

 by a thread at one end of a balance, and 

 counterpoised by a leaden weight, sus- 

 pended in the same manner, at the other. 

 Let this balance be hung to the inside of 

 the top of a large receiver ; which being 

 set on the pump, and the air exhausted, 

 the cork will preponderate, and show 

 itself to be heavier than the lead ; but 

 upon letting in the air again, the equili- 

 brium will be restored. The reason of 

 this is, that since the air is a fluid, and 

 all bodies lose as much of their absolute 

 weight in it as is equal to the weight of 

 their bulk of the fluid, the cork, being the 

 larger body, loses more of its real weight 

 than the lead does ; and therefore must in 

 fact be heavier, to balance it under the 

 disadvantage of losing some of its weight, 

 which disadvantage being taken off by re- 

 moving the air, the bodies then gravitate 

 according to their real quantities of mat- 

 ter, and the cork which balanced the 

 lead in air, shews itself to be heavier 

 when in vacuo. 



Set a lighted candle upon the pump, 

 and cover it with a tall receiver. If the 

 receiver holds a gallon, the candle will 

 burn a minute ; and then, having gradu- 

 ally decayed from the first instant, it will 

 go out; which shows, that a constant 

 supply of fresh air is as necessary to feed 

 flame, as animal life. 



The moment when the candle goes out, 

 the smoke will be seen to ascend to the 

 top of the receiver, and there it will form 

 a sort of cloud ; but upon exhausting the 

 air, the smoke will fall down to the bot- 

 tom of the receiver, and leave it as clear 

 at the top as it was before it was set upon 

 the pump. This shows, that smoke does 

 not ascend on account of its being posi- 

 tively light, but because it is lighter than 

 air; and its falling to the bottom, when 

 the air is taken away, shows that it is not 

 destitute of weight. So most sorts of 

 wood ascend or swim in water ; and yet 

 there are none who doubt of the wood's 

 having gravity or weight. 



Set a receiver, which is open at top, on 

 the air-pump, and cover it with a brass 

 plate and wet leather ; and having ex- 

 hausted it of air, let the air in again at 

 top through an iron pipe, making it pass 

 through a charcoal flame at the end of the 

 pipe ; and when the receiver is full of that 

 air, lift up the cover, and let down a 

 mouse or bird into the receiver, and the 

 burnt air will immediately kill it. If a 

 caudle be let clown into that air, it will go 



out directly ; but by letting it down gent- 

 ly, it will drive out the impure air, and 

 good air will get in. 



Set a bell on the pump-plate, having a 

 contrivance so as to ring it at pleasure, 

 and cover it with a receiver ; then make 

 the clapper strike against the bell, and 

 the sound will be very well heard; but, 

 exhaust the receiver of air, and then, it' 

 the clapper be made to strike ever so 

 hard against the bell, it will make no 

 sound ; which shows, that air is absolute- 

 ly necessary for the propagation of sound. 



It has been shown, that air can be ra- 

 refied, or made to expand ; we now pro- 

 ceed to show, that it can also be condens- 

 ed, or pressed into less space than what 

 it generally occupies. The instrument 

 used for this purpose is called a conden- 

 ser : (fig. 9) represents a machine of this 

 kind ; it consists of a brass barrel, con- 

 taining a piston, which has a valve open, 

 ing downwards; so that as the piston is 

 raised, the air passes through the valve ; 

 but as the piston is pushed down the air 

 cannot return, and is, therefore, forced 

 through a valve at the bottom of the bar- 

 rel, that allows it to pass through into the 

 receiver, U, but prevents it from return- 

 ing. Thus, at every stroke of the piston, 

 more air is thrown into the receiver, 

 which is of very thick and strong glass. 

 The receiver is held down upon the plate, 

 C, by the cross piece, D, and the screws, 

 EF. The air is let out of the receiver by 

 the cock, G, which communicates with it. 



The sound of a bell is much louder in. 

 condensed than in common air. A phial 

 that would bear the pressure of the com- 

 mon atmosphere, when the air is exhaust- 

 ed from the inside, will be broken by con- 

 densing the air round it. These experi- 

 ments may be made under the receiver B. 



A very beautiful fountain may be made 

 by condensed air. Procure a strong cop- 

 per vessel, (fig. 10) having a tube that 

 screws into the neck of it, so as to be air- 

 tight, and long enough to reach near to 

 the bottom. Having poured a quantity 

 of water into the vessel, but not enough 

 to fill it, and screwed in the tube, adapt 

 to it a condensing syringe, and condense 

 the air in the vessel ; shut the stop-cock, 

 and unscrew the syringe, then, on open- 

 ing the stop-cock, the air acting upon the 

 water in the vessel, will force it out into 

 a jet of very great height. A number of 

 different kinds of jets may be screwed on 

 the tube, such as stars, wheels, &c. form- 

 ing a very pleasing appearance. 



The air-gun is a pneumatical instru- 

 ment, of an ingenious contrivance, which 



