78 Correction in the Meteorological Essays. 



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 the hy- 

 grometer ; 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 temperature, 

 we must look for the amount of the alteration of volume due to 

 the heat in the second column, and for the expansion due to the 

 vapour in the third column. Add these together, if they have 

 like signs ; or subtract one from the other if they have different 

 signs. As the volume corrected by this quantity is to the ori- 

 ginal volume, so is the standard specific gravity to the specific 

 gravity, as affected by the expansion or contraction. To this 

 must be added the increase of weight due to the vapour in the 

 fourth column, and the result will be the correct specific gravity 

 sought. 



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 op* 

 posite to 60° the number +05833, and in the third column op* 

 posite to 40° we have + .00934 : the sum of which is + .06767 ; 

 therefore 



1.06767 : l :: i : 9366s. 



In the fourth column opposite to 40° we find +.00580, and 

 .93668 + .00580 = .94248 ; 

 which is the correct specific gravity under the assumed circum- 

 stances. 



Art. XIII. On the Barometer- By J. F. Daniell, P.R.S. 



The following paper requires a few words of preface. It contains, 

 as far as I am able to recollect, the substance of a communication 

 which I had the honour to make to the Royal Society, in the 

 month of November last, before the commencement of their last 



