PRESSURE OF THE AIR 



123 



some of the air has been removed. As the atmospheric pres- 

 sure varies, the top of the box rises or falls, and this motion is 

 communicated by a spring and levers to a hand moving over 

 a scale, which indicates the pressure. Since this style of 

 barometer is easily carried about, it is frequently used in 

 estimating the height of mountains, etc. The barograph is 

 essentially an aneroid barometer with an indicator adapted 

 to making a continuous record on a moving strip of paper. 



FIG. 42. Lift pump. (Duff.) FIG. 43. Force pump. (Duff.) 



107. Lift Pumps and Siphons. The ordinary lift pump for 

 raising water and other liquids above their natural level, 

 makes use of the pressure of the air. The pump has a plunger 

 and valves by means of which the air is removed from the 

 pipe extending down into the liquid, and the pressure of the 

 air then forces the liquid to rise in the pipe. Liquids will not 

 rise to any height, however, for the air only presses upward with 

 a force of about 15 pounds to the square inch. At sea level, 



