PNEUMATICS. 137 



much rarefied at the altitude of 500 miles, that it 

 would fill a sphere equal in diameter to the orbit 

 of Saturn. 



The elastic power of the air is always equivalent 

 to the force which compresses it; for if it were less, 

 it would yield to the pressure, and be more com- 

 pressed ; were it greater, it would not be so much 

 reduced, action and re-action being always equal ; 

 so that the elastic force of any small portion of the 

 air we breathe is equal to the weight of the incum- 

 bent part of the atmosphere, that weight being the 

 force which confines it to the dimensions it pos- 

 sesses. 



To prove this by an experiment, pour some 

 quicksilver into the small bottle A (Plate 6. fig. 5.) 

 and screw the brass collar c of the tube B C into 

 the brass neck of the bottle, and the lower end of 

 the tube will be immersed into the quicksilver, so 

 that the air above the quicksilver in the bottle will 

 be confined there. This tube is open at top, and 

 is covered by the receiver G, and large tube E F ; 

 which tube is fixed by brass collars to the receiver, 

 and is closed at top. This preparation being made, 

 exhaust the air out of the receiver G, and its tube, 

 by putting it upon the plate of the air-pump, and 

 the air will, by the same means, be exhausted out 

 of the inner tube B C, through its open top at C. 

 As the receiver and tubes are exhausting, the air 

 that is confined in the glass bottle A will press so 

 by its spring, as to raise the quicksilver in the inner 

 tube to the same height as it stands in the baro- 

 meter. 



Miscellaneous Experiments. 



1. Plate 6. fig. 6. represents a little machine, con- 

 sisting of two mills, a and b, which are of equal 



