PART III. 



MACHINERY IN CONNECTION WITH AIR 



CHAPTER I. 



PRESSURE OF AIR. 



Pneumatics treats of the me- 

 chanical properties of the air. 



The actual weight of the air 

 may be correctly found by weigh- 

 ing a strong glass vessel furnished 

 with a stop-cock, a (fig. 262), after 

 the air lias been withdrawn from 

 it by means of an air-pump. Let 

 it be accurately balanced by 

 weights in the opposite scale; 

 then turn the stoj>cock and admit 

 the air, and it will immediately 

 descend, as shown in the figure. 

 The weight of the admitted air 

 may be ascertained by adding 

 weights until it is again balanced. 



Fi?. S62. 



Balance for Weighing Air. 



HEIGHT AND WEIGHT OF THE ATMOSPHERE. 



The atmosphere which covers the earth extends upward 



to a height of about fifty or sixty miles. At the surface 



of the earth the air is about eight hundred times lighter 



than the water, and the higher we ascend, the rarer or 



239 



