A TMOSPHERIC PRESS USE. 



161 



sure produce corresponding variations in the height of the 

 barometer column. It is used to determine the height of 

 places above the sea-level, foretell storms, etc. When, at a 

 given place, the " barometer falls," a storm is generally 

 looked for. Sometimes the storm does not come, and 

 faith in the accuracy of the instrument is shaken. But, in 

 fact, the barometer did not announce a coming storm ; it 

 did proclaim a diminution of atmospheric pres- 

 sure from some cause or other. Its declarations are 

 perfectly reliable ; inferences from, those declarations are 

 subject to possible error. 



280. The Aneroid Barometer. This instrument consists 



of a cylindrical box of metal with a top 

 of thin, elastic, corrugated metal. The 

 air is removed from the box. The top 

 is pressed inward by an increased 

 atmospheric pressure ; whenever the 

 atmospheric pressure diminishes, it is 

 pressed outward by its own elasticity 

 aided by a spring beneath. These 

 movements of the cover are transmitted 

 and multiplied by a combination of 

 delicate levers. These levers act upon 

 an index which is thus made to move 

 over a graduated scale. Such barome- 

 ters are much more easily portable 

 than the mercurial instruments. They 

 are made so delicate that they show 

 a difference in atmospheric pressure 

 when transferred from an ordinary 

 table to the floor. Their very delicacy involves the necessity for care- 

 ful usage or frequent repairs. 



281. The Baroscope. Air, having weight, has 

 buoyant power. The Principle of Archimedes ( 238) 

 applies to gases as well as to liquids. Prom this it follows 

 that the weight of a body in air is not its true weight, but 

 that it is less than its true weight by exactly the weight of 



FIG. 96. 



