158 



THE REASON WHY. 



" Fair weather cometh out of the north : with God is terrible majesty." 

 JOB xxxvu. 



the alterations of the relative pressures of the air and the mercury 

 ;u-f more clearly denoted than ty an inspection of the mercury 



its, If. 



Fifc. 20. BAROMETER. 



Fig. 21. TTTBE OF BAROMETER, 

 WHEEL, AND PULLET. 



635. Why does the hand of the weather dial change its 

 position when the column of mercury rises or falls ? 



Because a weight, which floats upon the open surface of the 

 mercury, is attached to a string, having a nearly equal weight at 

 the other extremity ; the string is laid over a revolving pivot to 

 which the hand is fixed, and the friction of the string turns the 

 hand, as the mercury rises or falls. 





