20 



PNEUMATICS. 



piston valves are opened against the 

 atmospheric pressure, and however ac- 

 curately the barrels and pistons be con- 

 structed, yet there will necessarily be a 

 certain space, capable of containing air, 

 below the piston valve, when the piston 

 is at the bottom of the barrel. As soon as 

 the rarefaction has proceeded so far, that 

 the air which filled the barrel when the 

 piston was at the top, being reduced to 

 this last-mentioned space, acquires an 

 elasticity, exceeding the atmospheric 

 only by the force necessary to balance 

 the resistance of the valve, no more air 

 can be discharged by the piston. 



To calculate the power of such a 

 pump, let us suppose the space below 

 the valve, when the piston is at the bot- 

 tom of the barrel, to be the 1000th part 

 of (he whole capacity of the barrel ; and 

 let the force of the piston valve be the 

 100th part of the atmospheric pressure. 

 If the atmospheric density be 1000, the 

 density of the air under the piston valve, 

 when'at the bottom of the cylinder, at 

 the extreme limit of exhaustion, will be; 

 1010. When the piston has been raised, 

 this will be rarefied 1000 times, and 



therefore its density will be or 



= 1.01. The elasticity of this rarefied 

 air resists the exhausting valves V, V. 

 Let the density of the air which would 

 open these valves be the same as for the 

 piston valves, viz. 0.01. Hence the force 

 which resists the elasticity of the air in 

 the receiver, is 1.01 + 0.01 = 1.02. This 

 number will therefore express the densi- 

 ty of the air in the receiver, at the ex- 

 treme limit of rarefaction, that of the 

 atmosphere being 1000. The same prin- 

 ciples will evidently apply when any 

 other numbers are selected. 



'II. The Condenser. 



(38.) The condenser, as the name im- 

 plies, is the opposite of the air-pump. 

 R (fig. 21) is a receiver, with a valve 

 V in the neck, opening inwards. C is a 

 stop-cock in a tube connected with a 

 barrel in which a solid piston without a 

 valve plays air-tight. B is a small ori- 

 fice to admit air below the piston when 

 it is drawn above B. 



Suppose now the piston above B and 

 air filling ah 1 the apparatus, of the same 

 density as' the atmosphere : upon press- 

 ing the piston down, the air in the 

 pump-ban-el will be compressed after 

 the piston passes B, and will force 

 open the valve V ; and when the piston 



shall have arrived at the 



bottom of the ban-el it fig. 21. 



will be forced into the re- 

 ceiver, except that part 



which occupies the neck 



C. Every succeeding 



stroke of the piston will 



be attended with a similar 



effect, and thus the air in 



R will be continually con- 

 densed. [__ji; 



Neglecting the air con- 

 tained in the neck C, 



which is very small, the 



portion forced into the re 



ceiver at each stroke is the 



contents of the barrel B G 



at the atmospheric density. If^j 

 (39.) To indicate the || i 



degree of condensation 



which has been obtained, 



& gauge may be attached 



to the condenser. 



Let AB (Jig. 22) be a glass tube 



communicating at E with a vessel C 



containing mercury. This 



vessel is closed at the 



top, in which is inserted 



a tube communicating 



with the receiver of the fa 22 - 



condenser. The tube A B 



at first contains ah* of the 



atmospheric pressure, and 



consequently the level of 



the mercury in the tube is 



the same as in the cistern 



C. Let the tube be now 



closed at the top A, so as 

 to be air-tight, and let the 

 condensation be produced. 

 The increased pressure on 

 the surface of the mercury 

 in C will force mercury up 

 in the tube AB. 



Let. us suppose that the mercury is 

 raised to half the height of A above the 

 surface of the mercury in the cistern. 

 The air in the tube will thus be reduced 

 to half its bulk, and will therefore exert 

 double the pressure, or a force equal to 

 twice the atmospheric pressure. (30.) 

 This pressure, together with that of the 

 column of mercury in the tube AB, 

 above the level C, balances the pressure 

 of the condensed air in C. Hence the 

 pressure of the condensed air in this 

 case will be equal to that of a column 

 of mercury whose height is found by add- 

 ing the height of the mercury in A B, 

 above the level C, to twice the height of 

 the barometer. There will be no diffi- 

 culty in generalizing this principle. 



