726 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



the year is the same ; the isotlieral to that of points where the mean 

 temperature of the hottest months is the^ same ; and the isocliimenal 

 is the isotherm of the coldest months. All these means of tempera- 

 ture are calculated and reduced to the same altitude above the sea 

 level; but it is not to be forgotten that the altitude has a great 

 influence on these values. 



