Pneumatics. 71 



pump-plate, and the lower end immersed in 

 quicksilver at n in the vessel N. To this tube is 

 fitted a wooden ruler m m, called the gage, which 

 is divided into inches and parts of an inch, from 

 the bottom at n (where it is even with the sur- 

 face of the quicksilver), and continued up to 

 the top, a little below, to thirty or thirty-one 

 inches. 



As the air is pumped out of the receiver M, it 

 is likewise pumped out of the glass tube m n, be- 

 cause that tube opens into the receiver through 

 the pump-plate; and as the tube is gradually 

 emptied of air, the quicksilver in the vessel N is 

 forced up into the tube as in a barometer, by the 

 pressure of the atmosphere. And if the receiver 

 could be perfectly exhausted of air, the quick- 

 silver would stand as high in the tube as it does 

 at that time in the barometer : for it is supported 

 by the same power or weight of the atmosphere 

 in both. 



The quantity of air exhausted out of the re- 

 ceiver on each turn of the handle, is always pro- 

 portionable to the ascent of the quicksilver on 

 that turn ; and the quantity of air remaining in 

 the receiver, is proportionable to the defect of 

 the height of the quicksilver in the gage, from 

 what it is at that time in the barometer. 



By means of the air-pump all the mechanical 

 properties of air are, as before observed, most 

 completely ascertained. Thus the weight and 

 pressure are clearly proved by a very easy and ob- 



