160 THE EEASON WHY. 



' He caused an east wind to blow in the heaven ; and by his power he brought 

 in the south wind." PSALM ixxvm. 



701. Why does the barometer enable us to calculate the 

 "height of mountains ? 



Because, as the barometer is carried up a mountain, there is a 

 less depth, of atmosphere above to press upon the mercury ; it 

 therefore falls, and by comparing various observations, it has been 

 found practicable to calculate the height of mountains by the 

 fall of the mercury in a barometer. 



702. To what extent of variation is the weight of the 

 atmosphere liable ? 



It may vary as much as a pound and a half to the square 

 inch at the level of the sea. 



703. When does the barometer stand highest ? 



When there is a duration of frost, or when north-easterly winds 

 prevail. 



704. Why does the barometer stand highest at these times? 

 Because the atmosphere is exceedingly dry and dense, and fully 



balances the toeight of the column of mercury. 



705. When does the barometer stand lowest ? 



When a thaw follows a long frost ; or when south-west wind* 

 prevail. 



706. Why does the barometer stand lowest at those times ? 

 Because much moisture exists in the air, by which it is rendered 



less dense and heavy. 



707. What effect has heat upon the barometer ? 



It causes the mercury to fall, by evaporating moisture into the 

 air. 



708. What effect has cold upon the barometer ? 



It causes the mercury to rise, by checking evaporation, and 

 increasing the density of the air. 



In noting barometrical indications, more attention should be paid to tho 

 tendency of tho mercury at the time of the observation, than to the actual 

 ttate of the column, whether it stands high or low. The following rules of 

 tarometio -coding are given as generally accurate, but liable to exceptions :- 



