18 



PNEUMATICS. 



compressed ; for since the valve V 

 opens upwards, it cannot pass through 

 it. As soon as it is so far condensed, 

 that its elasticity exceeds the atmos- 

 pheric pressure, by a force sufficient to 

 raise the valve in the piston P , it will 

 pass into the atmosphere through that 

 valve, and will continue to pass into it, 

 until the piston P (Jig. 19.) strikes the 

 bottom of the barrel. 



fig. 19. 



The pistons now have assumed the 

 position (Jig. 19.): the wheel W is turned 

 in the opposite direction, so that P will 

 descend and P' will ascend. In this mo- 

 tion, the atmospheric pressure acts on 

 each piston, against the inferior force of 

 the rarefied air in the barrel, and there- 

 fore resists the ascent ; and the re- 

 sistance increases as the rarefaction 

 proceeds. These resistances are, how- 

 ever, not felt by the operator, since they 

 balance each other through the me- 



dium of the wheel and racks ; and this, 

 independently of the increased speed of 

 the process of rarefaction, is one of the 

 advantages of the double barrel. 



As the piston P descends, the rarefied 

 air in P V is condensed ; and as soon as 

 its elasticity exceeds the atmospheric 

 pressure by a force sufficient to open the 

 valve in the piston P, it will pass into 

 the atmosphere, and will be all dis- 

 charged when the piston shall arrive at 

 the bottom. 



While the piston P has been descend- 

 ing and discharging the air below it, 

 the piston P' has been ascending and 

 drawing more air from the receiver, 

 through the valve V. For as P' ascends, 

 it leaves a vacuum below it ; the elas- 

 ticity of the air in the receiver and tubes 

 encountering no resistance above the 

 valve V, opens it, and continues to pass 

 through until its elasticity exceeds that 

 of the air in the barrel, by a less force 

 than that which is sufficient to raise the 

 valve V. And in this way the process is 

 continued*. 



(37.) In such an instrument there is a 

 very obvious limit to the degree of rare- 

 faction. When the elasticity of the air 

 in the receiver is no longer sufficient to 

 open the valves V, V, it is clear that no 

 further rarefaction can be effected. Be- 

 sides, it is to be considered, that the 



* Fig. 20. (see next page) is a, perspective view of 

 this important instrument, which, though its parts are 

 somewhat differently arranged, is exactly the same in 

 principle, and marked with the same letters of rrfef- 

 ence. 



