154 PNEUMATICS. 



In using the air-pump, every substance contain- 

 ing moisture should be removed from the pump- 

 plate, as water assumes the form of an elastic 

 vapour when the pressure of the atmosphere is 

 taken away. The receivers used formerly to be 

 placed upon the pump-plate, on leathers soaked in 

 water or oil ; but Mr. Nairne discovered that an 

 elastic vapour arose from this that considerably 

 affected the gage, and prevented it from showing 

 the degree of rarefaction of the air. Instead of 

 placing leathers under the receivers, the best way 

 is, to have the pump-plate ground perfectly flat, as 

 also the edge of the receiver, which should be rub- 

 bed with a little hog's lard or soft pomatum ; this 

 wdll perfectly exclude the air, without affording 

 any moisture. The pump-plate and the receiver 

 should be wiped very clean. 



When leathers are used, the barometer-gages 

 will not show the degree of rarefaction of the air ; 

 which, however, may be ascertained by a gage in- 

 vented by Mr. Smeaton, and called, from its form, 

 the pear-gage. It consists of a glass vessel, in the 

 form of a pear (Plate 6. fig, 11.), and sufficient to 

 hold about half a pound of mercury : it is open at 

 one end, and at the other end is a tube hermeti- 

 cally closed at top. The tube is graduated, so as 

 to represent proportionate parts of the whole capa- 

 city. This gage, during the exhaustion of the 

 receiver, is suspended in it by a slip wire, over a cis- 

 tern of mercury, placed also in the receiver. When 

 the pump is worked as much as is thought neces- 

 sary, the gage is let down into the mercury, and 

 the air re-admitted. The mercury will immediately 

 rise in the gage ; but if any air remained in the 

 receiver, a certain portion of it would be in the 



