PART III. 



PNEUMATICS. 



CHAPTER I. 



PRESSURE OF AIR. 



Fig. 17 



Pneumatics treats of the mechanical properties of 

 the air. 



The actual weight of the 

 air may be correctly found by 

 weighing a strong glass ves- 

 sel furnished with a stop-cock, 

 a {Figure 177), after the air 

 has been withdrawn from it by 

 means of an air-pump. Let 

 it be accurately balanced by 

 weights in the opposite scale ; 

 then turn the stop-cock and ad- 

 mit the air, and it will imme- 

 diately descend, as shown in 

 the figure. The weight of the 

 admitted air may be ascertain- 

 ed by adding weights till it is 

 again balanced. 



Balance for Weighing Ai 



HEIGHT AND WEIGHT OF THE ATMOSPHERE. 



The atmosphere which covers the earth extends up- 

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



