

SPECIFIC GRAVITY AND PRESSURE. 245 



atmosphere, the weight of a column of mercury which is equal 

 to half the atmospheric pressure : the whole pressure upon the 

 enclosed air is -| times the atmospheric pressure, and the volume, 

 being diminished in the inverse proportion, is now of the original 

 volume. Now turn the tube round, and hold the closed end up- 

 permost, fig. 168 G. In this position the external pressure balances 

 both the weight of the mercury and the pressure of the air enclosed 

 at the top of the tube ; but as the mercurial column represents 

 half the pressure of the atmosphere, the enclosed air has now to 

 bear only the remaining half of the atmospheric pressure : it will 

 be found that in this position the volume of the air in the tube is 

 double of what it was originally. 



The specific gravity of air is altered by expansion 

 or compression. A litre of air, that is, 1,000 CC , weighs, 



under ordinary circumstances, - = l gr *25, the spe- 



o(JU 



cific gravity of air being -g-J-^. If the same air be ex- 

 posed to double the pressure, its volume will be 500 CC , 



and i- will now weih * = 0-0025 = = 





gramme; the specific gravity of the air has, therefore, 

 been doubled. Hence generally, while the volume of a 

 gas is inversely proportional to the pressure, the specific 

 gravity of a gas is directly proportional to the pressure. 



The pressure of the atmosphere becomes less as the 



.height above the level of the sea increases, and the 



1 specific gravity of the air becomes therefore less the 



i higher we ascend in the atmosphere. The rule given 



on page 233 for estimating the elevation above the 



'surface by the depression of the mercurial column is 



| therefore not strictly correct, and is only approximately 



'correct for inconsiderable heights. For although a 



column of air on the surface ll m high exerts a 



'pressure which is equal to that of a column of mercury 



l mm high, it will at a greater elevation, in consequence 



of the gradual diminution of the specific gravity, require 



