424 ] 



C H A P. IV. 



Of the Denfity of the Atmofphere. 



Denftty is the relation which the quantity of matter in any fub- 

 ftance bears to the bulk of that fubftance. It might as well have 

 been faid, that it is the relation that the bulk of any fubilance 

 bears to the quantity of matter contained in it, had it not been, 

 that tlie quantity of matter remains unaltered while the bulk varies.. 

 As the quantity of matter in all ponderable fubftances is propor- 

 tioned to the weigh/ and is no otherwife difcoverable, the weight 

 is often fubflituted for the quantity of matter. Hence the ma/s of 

 any body is equal to its denfity multiplied into its bulk. 



To compare with precifion the bulk and quantity of matter in 



any fubftance, it is neceffary that a given weight and a given bulk 



be afligned, in which the relation betwixt them is afcertained and 



exprefled in known weights and meafures, which may ferve as a 



fiandard in all other cafes. 



The bulk of all elaftic fluids is afFe£led and varied both by a va- 

 riation oi prejjion and a variation of temperature ; to aflign therefore 

 a general ftandard for the denfity of ihefe fluids, it is neceffary to 



difcover- 



