302 PRESSURE IN THE AIR, 



ments, improperly called barometers* under 

 different states of atmospherical influence ; the 

 one under a glass receiver from which the exter- 

 nal air is entirely excluded, containing* any 

 quantity of air, say 2 cubic feet, weighing 25 

 or 30 grains ; the other in open space, exposed 

 to the influence of the whole atmospherical 

 column; it must evidently follow, that if the 

 elevation of the mercury in the tube be caused 

 by weight, by the pressure of weight of the at- 

 mosphere ; the mercury in the tube of each 

 instrument ought to undergo unequal degrees 

 of elevation ; so far, however, from the ele- 

 vation being unequal, the mercury will be 

 found to preserve the same parallel in both. 

 The preservation of the mercury to the same 

 parallel in two instruments placed in situations 

 so totally different from each other, either proves 



* Properly speaking, a barometer is a measurer of weight 

 only, and consists of a pair of scales with a weight in one 

 balance, with a view of measuring the pressure of weight 

 produced by the matter contained in the other ; and the baro- 

 meter, as now constructed, may be employed for the same 

 purpose; it may be employed also as an excellent measure, 

 for ascertaining the relative effect produced by weight, and 

 the pressure produced by expansibility, as well as between 

 the relative degrees of expansible pressure, which are exerted 

 by different gaseous fluids. In every case, the degree of ele- 

 vation of the mercury in the tube will be the sum of the 

 pressure produced. 



