PNEUMATICS, 



ECTfON I. 



THE ATMOSPHERE AND ATMOSPHERIC PRESSURE. 



268. What is Pneumatics ? Pneumatics is 

 that branch of Physics which treats of aeriform 

 bodies, their mechanical properties, and the ma- 

 chines by ivhich they are used. 



269. Tension of Gases. However small their 

 quantity , gases always fill the vessels in which they 



are held- If a bladder or India rub- 

 ber bag, partly filled with air, and 

 having the opening well closed, be 

 placed under the receiver of an air- 

 pump, the bladder or bag will be fully 

 distended, as shown in the figure, 

 when the air surrounding the bladder 



,, is pumped out. The flexible walls 



FIG. 93. 



are pushed out by the air confined 

 within. This tendency is called elastic force or tension. 



270. The Type. As water was, for obvious reasons, 

 taken as the type of liquids, so atmospheric air will be 



