70 PNEUMATICS. 



Barometer. 



A machine for measuring the weight and varia- 

 tions of the atmosphere. 



Barometric Changes. 



The mercury in the barometer is seldom to be 

 seen lower than 28 inches, or higher than 30^. It 

 indicates as follows : 



31 inches - Very dry weather hard frost. 



30| - - - Settled fair settled frost. 



30 - - - Fair frost. 



29 J - - - Changeable. 



29 - - - Rain snow. 



28 1 - - - Much rain much snow, 



28 - - - Stormy weather. 

 In January, 1822, the mercury fell to 2810, which 

 is said to have been the lowest ever remarked in 

 England. By the fall, a pressure equal to 2000 Ibs. 

 was removed from each person's body, yet probably 

 no individual was at the time conscious of it. The 

 maximum height of the mercurial column is about 

 nine o'clock A.M. ; the mean at twelve, and the 

 minimum at three P.M. If the column rise from 

 nine A.M. to three P.M. it indicates fine weather ; if 

 it fall during that interval, rain may be expected. 



The Wheel Barometer of the form of a Syphon. 



A small float of iron or glass rests on the exterior 

 surface of the mercury, suspended by a slender 

 thread, passed round a small wheel or cylindrical 

 axis that carries the index. By this construction the 

 variation of the height of the mercurial column is 

 reduced to half the ordinary measure, yet the cir- 

 cumference traversed by the needle is greatly aug- 

 mented. 



The Common Barometer. 



A tube, a quarter of an inch in diameter, is a good 



