ATE PUMP. 



barometer. The atmosphere presses on the surface of the- 

 mercury in the cistern M, while the column of mercury in 

 the tube H G is pressed upon by the rarefied air in E. The 

 height of the column, therefore, sustained in the tube, indicates 

 the difference between the pressure of the external air and the air 

 in the receiver. 



When a gauge, of the form represented in fig. 6, is used, it 

 is necessary that it should have the height of about 30 inches, 

 since, when a high degree of rarefaction has been effected, a 

 column of mercury will be sustained in the tube H G, very little 

 less than in the common barometer. In small pumps, where 

 this height would be inconvenient, a siphon-gauge, such as that 

 represented in fig. 7, is used. This gauge is screwed on to a pipe 

 communicating with the receiver. Mercury fills the leg A B, 

 which is closed at the top A, and partially fills the legs s. 

 "When the atmosphere communicates freely with the tube D c, 

 the surface of the mercury in s being pressed by its 

 full force, sustains all the mercury which the tube 

 B A can contain, and this tube, consequently, 

 remains completely filled ; but when the pipe DCS 

 is put in communication with the exhausted receiver, 

 the surface of the mercury in s being acted upon only 

 by the pressure of the rarefied air in the receiver, 

 the weight of the higher column in B A will pre- 

 dominate, and the mercury will fall in it, until the 

 difference of the levels in the two legs shall be equal 

 to the pressure of rarefied air in the receiver. 



29. The condensing syringe differs from the ex- 

 hausting syringe only in the direction in which the 



valves are placed. It consists of a cylinder and piston, as repre- 

 sented in fig. 5. When the piston is drawn upwards, the cock 

 D is open, and c is closed, and the cylinder is filled with air 

 proceeding from the external atmosphere. ; When the piston is 

 pressed downwards, the cock D is closed and c is opened, and 

 the air which filled the cylinder is forced into the vessel E. On 

 raising the piston again, the cock c is closed and D is opened, 

 and the effects take place as before. It is evident that, by every 

 stroke of the piston, as much air as fills the cylinder is driven 

 into the vessel E. 



In practice, the cocks D and c are replaced by two valves, one 

 in the bottom of the cylinder, and the other in the piston, both 

 opening downwards, contrary to the valves in the exhausting 

 syringe. 



The operation is explained in the same manner. 



30. The condenser is an apparatus which bears to the con- 



Hi 



