THE WEIGHT OF THE AIR 



381 



pressure at the floor level. Aneroid barometers are frequently 

 made no larger than a watch and can be carried conveniently 

 in the pocket, but they get out of order easily and must be 

 frequently readjusted. The aneroid barometer is an air-tight 

 box whose top is made of a thin metallic 

 disk which bends inward or outward ac- 

 cording to the pressure of the atmos- 

 phere. If the atmospheric pressure in- 

 creases, the thin disk is pushed slightly 

 inward; if, on the other hand, the at- 

 mospheric pressure decreases, the pres- 

 sure on the metallic disk decreases and 

 the disk is not pressed so far inward (Fig. 

 251). The motion of the disk is small, 

 and it would be impossible to calculate FIG. 251. Principle of 

 changes in atmospheric pressure without ^ aneroid barometer - 

 some mechanical device to magnify the slight changes in 

 motion. 



In order to magnify the slight changes in the position of the 

 disk, the thin face is connected with a system of levers, or wheels, 

 which multiply the changes in motion and communicate them 

 to a pointer which moves around a graduated circular face. 



The weight of the air. We have seen that the pressure 

 of the atmosphere at any point is due to the weight of the air 

 column which stretches from that point far up into the sky 

 above. This weight varies slightly from time to time and 

 from place to place, but it is equal to about 15 pounds to the 

 square inch, as shown by actual measurement. It comes to 

 us as a surprise sometimes that air actually has weight; for 

 example, a mass of 12 cubic feet of air at average pressure 

 weighs i pound, and the air in a large assembly hall weighs 

 more than i ton. 



We are practically never conscious of this really enormous 



