204 OBSERVATIONS ON THE MEAN TEMPERATURE 



The beautiful memoir of Humboldt on the Isothermal Lines, 

 or lines of equal temperature, and on the distribution of heat 

 over the globe *, has given a fresh impulse to this fundamental 

 branch of Meteorology, and will, no doubt, introduce a new 

 degree of precision into those loose and indefinite records of 

 temperature which have been so generally accumulated in 

 every part of Europe. In attempting to reconcile the for- 

 mula of Mayer with the observed results as given by this ce- 

 lebrated traveller, I expected to succeed, at least for the western 

 regions of the Old World, by the adoption of more correct 

 co-efficients ; but as I proceeded in the inquiry, I saw that the 

 principle of the formula was entirely irreconcileable with well 

 established facts, and I therefore sought for a law different 

 from the duplicate ratio of the sines. 



In comparing the temperature of the Equator with that of 

 45°, and with that of the highest latitude in Humboldt's se- 

 ries, it was obvious that the cold increased much more rapidly 

 towards the poles than had been believed ; and upon extend- 

 ing the comparison to the intermediate temperatures, I found 

 that the mean heat of any place was well represented by the 

 radius of its parallel of latitude, or, in geometrical language, 

 that the temperatures varied with the cosine of the latitude. 

 In expressing this law I have assumed 81^° as the mean 

 temperature of the Equator, the very same number which 

 Humboldt has preferred as the mean of various results under 

 distant meridians. The formula therefore becomes 



T = 81i Cos. Lat. 



The 



* This Memoir has been translated into English, and published in the Edin- 

 burgh Philosophical Journal, vol. iii. andiv. 



