168 AIR-PUMP. 



7. Eight grains of air are enclosed in a rigid vessel of such size 

 that the tension is 18^ pounds per square inch. What will be the 

 tension if three more grains of air be introduced ? 



Recapitulation. In this section we have considered 

 the Equality of tension and pressure, with several Ex- 

 perimental Illustrations; Mariotte's Law; 

 the Verification of that law for Compressed 

 and for Rarefied Gases; a brief Conclusion from 

 the teachings of these experiments. 



XgafcECTION III, 



AIR-PUMPS. LIFTING AND FORCE-PUMPS. 

 SIPHON. 



288. The Air-Pump. TJie air-pump is an 

 instrument for removing air from a closed vessel. 

 The essential parts are shown in section by Fig. 102 ; the 

 complete instrument, as made by Ritchie, is represented 

 by Fig. 103. 



The closed vessel R is called a receiver. It fits accu- 

 rately upon a horizontal plate, through the centre of which 

 is an opening communicating, by means of a bent tube, /, 

 with a cylinder, C. An accurately fitting piston moves in 

 this cylinder. At the junction of the bent tube with the 

 cylinder, and in the piston, are two valves, v and v' y open- 

 ing from the receiver but not toward it. The tension of 

 the air in R, and the pressure of the air upon the valves, 

 are equal. When the piston is raised, v' closes and the 

 atmospheric pressure is removed from v. The tension of 

 the air in R opens v. By virtue of its power of indefinite 



