

CLIMATE. 



21st of June, called the Spring, having generally a much lower 

 temperature than the season from the 21st of June to the 21st of 

 September, called the Summer. 



Let us see, then, whether we cannot render evident the cause 

 of this. 



The temperature of the weather in a given place, depends not 

 exclusively upon the thermal influence exercised by the sun 

 during each day. It must be remembered that when the days are 

 much longer than the nights, and the sun rises to a considerable 

 altitude, a greater quantity of heat is imparted to the atmosphere 

 and to all objects upon the surface during the day, than is lost 

 during the night, and, consequently, an increment of heat is 

 given to all such objects every twenty -four hours. The conse- 

 quence is, that the general effect of the sun's thermal influence 

 during each successive twenty-four hours is to augment the 

 temperature, and thus to increase by accumulation the heat from 

 da}- to day ; and this daily increase will obviously continue until, 

 by the shortening of the days and the decrease of the sun's 

 altitude, the increment of heat during the day becomes equal to 

 its decrement during the night. The day on which that takes 

 place will be the hottest day, because it will be that upon which 

 the daily accumulation will cease. After this, the days being 

 further shortened, and the sun's altitude further diminished, the 

 increment of heat during the day will be less than its decrement 

 during the night, and after each interval of twenty-four hours 

 there will be on the whole a decrease of heat, and so the tempe- 

 rature of the weather will be diminished. 



162. Now, from a due consideration of these circumstances, it 

 will be easy to see why the season of summer is warmer than the 

 season of spring, although the sun's altitude and the length of 

 the days are, on the whole, precisely the same both in one season 

 and the other, only succeeding each other in a contrary order. 

 Until the 21st of June the daily thermal influence of the sun con- 

 tinually increases, for the reasons just explained, and it is greater 

 on the 21st of June than on any other day before or after. But 

 although this thermal influence decreases after the 21st of June it is 

 still considerable, and from day to day adds something more or less 

 to the heat already accumulated in the atmosphere, and conse- 

 quently continues to augment the temperature ; and this increase 

 only ceases, when the thermal action of the sun during the day, 

 begins to be counteracted and balanced by the loss of heat during 

 the night. 



163. Hence it arises that a certain interval, from the 21st of 

 June to the latter part of July, is generally the hottest part of 

 the summer, being called the Dog-days, either because of the 



169 



