TRANSACTIONS OF THE SECTIONS. 



409 



perature. If we consider it in relation to the mean tempera- 

 ture, we sliall find a near coincidence between the fullowing 



aL+ a — 

 formula, and observation : m ■=. t' t"^, where a — the 



ascertained value of m, for the whole year, t the mean tempe- 

 rature of the year, and t' that of the particular period under 

 consideration. 



10. Secondly, it is obvious that the relation between the 

 values of m and the dryness of the air is inverse. This dry- 

 ness is usually expressed by the difference between the mean 

 temperature and the mean dew point, and where the latter is 

 perfectly determined, no better plan perhaps can be suggested. 

 But this is the case for very few places in Great Britain. There 

 is, however, another mode of expressing the dryness of the air, 

 which is fortunately applicable to the present purpose ; the 

 mean range of daily temperature, or mean difference of maxima 

 and minima, is a good approximation to an accurate ex23ression 

 of the relative dryness of the air. The following table of the 

 mean ranges of temperature near York has been determined 

 with the greatest nicety, by long averages, from the careful 

 and continued observations of Francis Cholmeley, Esq., of 

 Brandsby. 



January mean range. . . . 8'0 



February 10*1 



March 13-1 



April 16-2 



May 19-7 



June 20-1 



General mean range 



July mean range 19*6 



August 17'7 



September 16'0 



October 11-8 



November 9-0 



December 7*7 



14-08 



On comparing these numbers with Mr. Daniell's estimates of 

 the dryness of the air in London, they will be found analogous 



