of Barometrical Mensuration. 87 



To find the specific gravity of any mixture of air and aqueous 

 vapour by means of this table, we must proceed as follows : 



Note the temperature and the point of condensation by means 

 of the hygrometer : if they coincide, that is to say, if the air be 

 in a state of saturation, we shall find the specific gravity required 

 in the fifth column opposite to the proper degree of heat in the 

 first column. If the point of condensation be below the tempe- 

 rature, we must look for the correction to be applied separately 

 for the heat in the second column. The quantity to be sub_ 

 tracted for the vapour of the proper degree must be sought for 

 in the third column, and this must be applied mmw* the quantity 

 due to its weight, which stands beside it in the fourth. 



For example : if we wish to know the specific gravity of a 

 mixture of air and vapour of the temperature of 60°, and of 

 which the dew point is 40°, we find in the second column 

 opposite to 60° the number .05833, which, deducted from 

 1.00000, leaves .94167. In the second column, opposite to 40°, 

 we have .00876, and beside it in the third .00531. Now .00876 

 —.00531 =.00345, which subtracted from .94167, leaves .93822 

 as the number sought for. 



The application of this table to barometrical mensurations is 

 sufficiently simple. For this purpose, with the usual operations 

 at the upper and lower stations, must be combined simultaneous 

 observations of the dew point by means of the hygrometer, and 

 the approximate height may be corrected by the specific gravity 

 of the atmosphere so obtained. As the specific gravity of the 

 air at the time of the experiment is to 100000 the standard, so 

 will the approximate height be to the real height. 



I have not had the advantage of an opportunity of making 

 any trials practically to prove the superior accuracy of this 

 correction, but the theoretical reasons in its favour are con- 

 clusive, and to these I may add two or three particulars derived 

 from experience. 



General Roy, from his own experiments, as well as from a 

 careful review of those of De Luc, fixed the medium expansioji 

 of air for 1°=:.Q0245, and Sir George Shuckburgh assigned 



