158 



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 

 are more dearly denoted vijw by an inspection of the mercury 



FiR. 20. BAEOMETEE. Fig. 21. TUBE OF BAEOMETEB, 

 WHEEL, AND PULLEr. 



695. Why does tlie hand of the weather dial change if* 

 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 th 

 land, at the mercury rises or falls. 



