PNEUMATICS. 



being unresisted by any equivalent pres- 

 sure, the liquid is forced out. It should 

 be remembered, that the atmospheric 

 pressure on the surface E, is resisted by 

 an equivalent pressure on the surface of 

 the liquid in the vessel in which the leg 

 D is immersed. After the level of the 

 liquid in that vessel has fallen to D, the 

 liquid will cease to flow from the sy- 

 phon, which, therefore, remains full, 

 and may be hung up by a loop at B, till 

 again required for use. 



V. Of Aerostats, or Air-balloons. 



(51.) Aerostats, or air-balloons, are 

 machines, constructed so as to be able 

 to rise in the atmosphere, and float in it 

 at considerable heights, bearing with 

 them, in a car suspended from them, the 

 aeronaut. 



i The principle of the air-balloon is 

 exactly the same as that which governs 

 the ascent of a piece of cork from the 

 bottom of a vessel of water to its surface. 

 If any body is placed in a fluid, whether 

 elastic or inelastic, a gas or a liquid, it 

 will rise or sink, according as it is lighter 

 or heavier, bulk for bulk, than the fluid. 



Let AB (fig. 30.) represent a level 

 plane, and let C D represent the highest 



,fa 30 



stratum of th& atmosphere, andE F any 

 inferior stratum. Every part of the level 

 EFmust be equally pressed by the weight 

 of the incumbent atmosphere ; and, by 

 the general property of fluids, the pres- 

 sure to which it is submitted, it trans- 

 mits equally in every direction ; so that 

 a square inch of the level E F, is equally 

 pressed upwards and downwards by a 

 force equal to the weight of a column 

 of the atmosphere whose base is a square 

 inch at the level E F, and whose height 

 is the difference of the levels EF and CD. 

 Now, if a body G H I K, whose base is 

 a square inch, be placed with its base on 

 the level E F, it will take the place of as 

 much air as is equal to its own bulk. If 

 it be lighter than the air it has displaced, 

 it will press on the level E F with a less 

 force ; but the level E F will press on it 

 with the same force as before; and, 

 therefore, being pressed upwards with a 



greater force than downwards, it will 

 rise. If, on the other hand, it be heavier 

 than the air it displaces, it will fall ; and, 

 finally, if it be equal in weight with the 

 air it has displaced, it will remain sus- 

 pended : these last cases being establish- 

 ed by the same reasoning as the first. 



The air-balloon is a light silken bag 

 filled with a gas, which, bulk for bulk, is 

 lighter than air, so that when inflated, 

 the machine becomes considerably 

 lighter than the air which it displaces. 

 It will therefore ascend in the atmos- 

 phere with a force equal to the differ- 

 ence between its own weight and that of 

 the air it displaces. This difference, if 

 the balloon be sufficiently large, is so 

 considerable, that it is enabled to cany 

 up with it one or two persons in a car 

 attached to it. 



As it ascends, the air becoming less 

 dense (23.), the difference between its 

 weight and that of the air displaced by 

 it, is gradually diminished, until it attains 

 such an height, that the air it displaces 

 is so rare as to be only equal in weight 

 to the balloon. This, therefore, must 

 be the limit of its ascent. 



The aeronaut can descend by permit- 

 ting some of the gas to escape through 

 a valve, and thereby diminishing the 

 bulk of the balloon. By this means the 

 air it displaces is diminished, and the 

 weight of the balloon is made to exceed 

 that of an equal bulk of air ; and there- 

 fore it falls until it comes to a lower and 

 denser stratum, in which the weight of 

 the air, bulk for bulk, is equal to that of 

 the balloon, and here again it is sus- 

 pended. 



To be enabled to rise, the aeronaut is 

 provided with ballast, composed of bags 

 of sand ; upon throwing out some of these 

 he lightens the machine, and accordingly 

 rises. By these means, as long as a suf- 

 ficient quantity of gas remains in the 

 balloon, he can ascend and descend at 

 pleasure. 



VI. Of the Air-gun. 



(52.) The air-gun is an instrument for 

 projecting balls, or other missiles, by the 

 elastic force of condensed air. 



The principle of the air-gun is easily 

 understood. By means of a condenser, 

 such as has been described in (38.), air 

 is highly condensed in a strong receiver, 

 provided for the purpose, having a valve 

 in it which opens inwards. This receiver, 

 or magazine of compressed air, is screw- 

 ed upon the stock of the air-gun, so that 

 a communication can be made between 



