182 THE CHEMISTRY OF CREATION. 



we have spoken, presses upon the surface of the 

 mercury in the cup, and counterbalances it, so 

 preventing its running out Should the weight 

 of this column of air be by any means dimi- 

 nished, the fact will be immediately exhibited 

 by the mercury falling ; should it be increased, 

 it will rise. In other words, when the weight 

 of the column is greater, it counterbalances 

 more, and when lower, less mercury in the tube. 

 An ascending current of air rarified, and there- 

 fore made lighter by heat, causes the ebb of the 

 daily aerial tide in the tropics beforementioned ; 

 and this ebb is duly registered by the mercury in 

 the tube, standing at a lower mark. 



Elevation produces a similar depression, the 

 barometer falling gradually for every step of 

 ascent gained ;* and if it were possible to rise 

 to the very surface of the atmosphere, not a 

 drop of the mercury would remain in the tube. 

 This fact is of the utmost importance in the 

 admeasurement of heights, and proper formulae 

 exist by which it is easy to ascertain, after 

 certain corrections are made, the amount of 

 perpendicular elevation possessed by a moun- 

 tain, or other eminence. On the contrary, 



* The ingenious instrument called the anezoid barometer, 

 a pocket weather-register, consisting of an exhausted receiver 

 with a moveable lid and index, has proved very useful in 

 these investigations. 



