PNEUMATICS. 155 



gage ; and as it would occupy the top of the tube 

 above the mercury, it would show, by its size, the 

 degree of exhaustion : for the bubble of air would 

 be to the whole contents of the gage, as the quan- 

 tity of air in the exhausted receiver would to an 

 equal volume of the common atmospheric air. If 

 the receiver contained any elastic vapour gene- 

 rated during the rarefaction, it would be con- 

 densed upon the re-admission of the atmospheric 

 air, as it cannot subsist in the usual pressure. The 

 pear-gage, therefore, shows the true quantity of 

 atmospheric air left in the receiver. Hence, it will 

 sometimes indicate that all the permanent air is 

 exhausted from the receiver, except about tw^to 

 part, when the other gages do not show a degree 

 of exhaustion of more than 200 times, and some- 

 times much less. 



Particular care should be taken, after making 

 any experiments where vapour has been generated, 

 to clear the pump of it, before any other experi- 

 ments are attempted ; for the vapour remains not 

 only in the receiver, but also in the tubes and 

 barrels of the pump, and will, when the air is 

 again rarefied, expand as before. To clear the 

 pump of this vapour, take a large receiver, and, 

 wiping it very dry, exhaust it as far as possible. 

 The expansible vapour which remained in the 

 barrels and the pipes will now be diffused through 

 the receiver, and, consequently, will be as much 

 rarer than it was before as the aggregate capacity 

 of the receiver is larger than that of the pump and 

 pipes. If the receiver be large, one exhaustion 

 will be sufficient to clear the pump so far, that 

 what remains can be of no consequence. If the 

 receiver be small, the operation should be repeated 

 two or three times. 



