130 SPECIFIC GKAriTIES. 



SCH;. 2. Formula (16) has been obtained on the supposi- 

 tion that the temperature of the air remains constant in 

 passing from the lower to the higher station ; if, however, 

 the difference between the heights be very great, a consid- 

 erable error may be thus introduced, and formulae have 

 therefore been constructed in which account is taken, on 

 various hypotheses, of the variation of atmospheric temper- 

 ature. A formula of this kind is given in Lindeman's 

 Barometric Tables, constructed on the supposition that the 

 temperature diminishes in harmonic progression through a 

 series of heights increasing in arithmetic progression. 



Also, we have assumed that the temperature of the 

 mercury in the barometer is the same as that of the air 

 surrounding it ; but in some cases, as for instance when ob- 

 servations are made in a balloon, the barometer may not 

 remain long enough in the same place to acquire the tem- 

 perature of the surrounding air. The temperature of the 

 mercury may be observed, however, by placing the bulb of a 

 thermometer in the cistern of the barometer, and the tem- 

 peratures thus obtained must be used in (10). (See Be- 

 sant's Hydromechanics, p. 121.) 



SPECIFIC GRAVITIES. 



Ratios of tfye Specific Gravities of different sub- 

 stances to that of water at 60. 



