A PORTABLE BAROMETER 



FlG. 45. Aneroid barometer. 



barometer is extremely sensitive ; indeed, it is so delicate that 

 it shows the slight difference between the pressure at the table 

 top and the pressure at the floor level, 

 whereas the mercury barometer would 

 indicate only a much greater variation 

 in atmospheric pressure. The 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 fre- 

 quently readjusted. The aneroid ba- 

 rometer is an air-tight box whose top 

 is made of a thin metallic disk which 

 bends inward or outward according 

 to the pressure of the atmosphere. If the atmospheric pressure 

 increases, the thin disk is pushed slightly inward ; if, on the 

 other hand, the atmospheric pressure decreases, the pressure on 

 the metallic disk decreases and the disk is not pressed so far 

 inward. The motion of the disk is small, and it would be impos- 

 sible to calculate changes in atmospheric 

 pressure from the motion of the disk, 

 without some mechanical device to make 

 the slight changes in motion perceptible. 



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 multiplies the changes in 

 motion and communicates them to a 

 pointer which moves around a graduated 

 circular face. In Figure 45 the real ba- 

 rometer is scarcely visible, being securely 

 inclosed in a metal case for protection ; the principle, however, 

 can be understood by reference to Figure 46. 



FIG. 46. Principle of 

 the aneroid barometer. 



