PNEUMATICS. 158 



some circumstances respecting it could not be un- 

 derstood before the barometer was explained. 



The gage, or instrument for measuring the de- 

 gree of rarefaction or exhaustion produced in the 

 receiver, is a necessary appendage to the air-pump. 

 If a barometer be included beneath the receiver, 

 the mercury will stand at the same height as in the 

 open air ; but when the receiver begins to be ex- 

 hausted, the mercury will descend, and rest at a 

 height which is, in its proportion to its former 

 height, as the spring of the air remaining in the 

 receiver is to its spring before exhaustion. Thus, 

 if the height of the mercury, after exhaustion, is 

 the thousandth part of what it was before, we say 

 that the air in the receiver is rarefied 1000 times. 

 On account of the inconvenience of including a 

 barometer in a receiver, a tube of six or eight 

 inches in length is filled with mercury, and inverted 

 in the same manner as the barometer. This 

 being included, answers the same purpose, with 

 no other difference, than that the mercury does 

 not begin to ascend till about three-fourths of 

 the air is exhausted : it is called the short ba- 

 rometer gage. It is generally placed detached, 

 but communicating with the receiver by a tube 

 concealed in the frame, as is represented at Plate 6. 

 fig. 1. Others place a tube of a greater length 

 than the barometer, with its lower end in a vessel 

 of mercury, exposed to the pressure of the air, 

 while its upper end communicates with the re- 

 ceiver. Here the mercury rises as the exhaustion 

 proceeds ; and the pressure of the remaining air is 

 shown by the difference between its height and 

 that of a barometer in the room : this is called the 

 long barometer gage. 



