Aimosphcre and the Earth. 



235 



The greatest cold was on the 18th and 19th January (— 39°.32 

 the mercury froze) ; the greatest heat on the 8th July (87°.8). 



To deduce from these observations the mean temperature of 

 the air in Kasan, we may assume, without fearing any great er- 

 ror, that the mean of the observations made at 9 a. m., corres- 

 pond to the mean temperature of the whole year ; as here we 

 arc only considering one year, which is certainly not sufficient, 

 exactly to determine the mean temperature of a place. It is 

 known that we ap})roach nearest to the true mean temperature, 

 when we take the mean of the maxima and minima ; but, that 

 this mean comes very near the mean of the observations at 

 9 A. M., wc-may convince ourselves of, at least for three months, 

 from the above table. We may also consult the works of Bou- 

 vard (Mem. de FAcad. des Sciences, 1824), and Hallstrom 

 (Poggendorf, b. 80, p. 373.) 



The mean temperature of the air in Kasan, for thv^' year 1898 

 (or, more correctly, from November 18£7 to November 1828), 

 is therefore 36°.5 Fahrenheit ; but it is observable that the year 

 1828 was a cold year: the true mean temperature of Kasan 

 nmst, therefore, be taken at somewhat higher. The mean tem- 

 perature of April is considerably higher than the mean of the 

 year ; that of October not only approaches very near the mean 

 of 1828, but also corresponds, as we shall immediately sec, al- 

 most exactly to the mean temperature of Kasan. 



