Hence it is that the heat of summer increases as we approach the equator. 

 The lower the latitude is, the greater will be the height to which the sun will 

 rise. The maridian altitude of the sun at the summer solstice being every- 

 where forty-six degrees and fifty-six minutes more than at the winter solstice, 

 the heating effect will be proportionately greater. 



But this is not the only cause which produces the greatly superior heat of 

 summer as compared with winter, especially in the higher latitudes. The 

 heating effect of the sun depends not alone on its altitude at midday; it also 

 depends on the length of time which it is above the horizon and belo\v it. 

 While the sun is above the horizon, it is continually imparting heat to the air 

 and to the surface of the earth ; and while it is below the horizon, the heat is 

 continually being dissipated. The longer, therefore other things being the 

 same the sun is above the horizon, and the shorter time it is below it, the 

 greater will be the amount of heat imparted to the earth every twenty-four 

 hours. Let us suppose that between sunrise and sunset, the sun, by its cal- 

 orific effect, imparts a certain amount of heat to the atmosphere and the sur- 

 face of the earth, and that from sunset to sunrise a certain amount of this heat j 

 is lost: the result of the action of the sun will be found by deducting the latter 

 from the former. 



Thus, then, it appears that the influence of the sun upon the seasons de- 

 pends as much upon the length of the days and nights as upon its altitude ; but 

 it so happens that one of these circumstances depends upon the other. The 

 greater the sun's meridional altitude is, the longer will be the days, and the 

 shorter the nights ; and the less it is, the longer will be the nights, and the 

 shorter the days. Thus both circumstances always conspire in producing 

 the increased temperature of summer, and the diminished temperature of 

 winter. 



A difficulty is sometimes felt when the operation of these causes is consid- 

 ered, in understanding how it happens that, notwithstanding what has been 

 stated, the 21st of June when the sun rises the highest, when the days are 

 longest and the nights shortest is not the hottest day, but that on the contrary, 

 the dog-days, as they are called, which comprise the hottest weather of the year, 

 occur in. August ; and in the same manner, the 2lst of December-^ when the 

 height to which the sun rises is least, the days shortest, and the nights longest 

 is not usually the coldest day, but that, on the other hand, the most inclem 

 ent weather occurs at a later period. 



To explain this, so far as it depends on the position of the sun and the 

 length of the days and nights, we are to consider the folio wing circum- 

 stances : 



As midsummer approaches, the gradual increase of the temperature of the 

 weather has been explained thus : The days being considerably longer than 

 the nights, the quantity of heat imparted by the sun during the day is greater 

 than the quantity lost during the night ; and the entire result during the twenty 

 four hours gives an increase of heat. As this augmentation takes place alter 

 each successive day and night, the general temperature continues to increase. 

 On the 21st of June, when the day is longest, and the night is shortest, and the 

 sun rises highest, this augmentation reaches its maximum ; but the temperature 

 of the weather does not therefore cease to increase. After the 21st of June, 

 there continues to be still a daily augmentation of heat, for the sun still con- 

 tinues to impart more heat during the day than is lost during the night. The 

 temperature of the weather will therefore only cease to increase when, by th> 

 diminished length of the day, the increased length of thu night, and the dimin- 

 ished meridional altitude of the sun, the heat imparted during the day is just 

 balanced by the heat lost during the night. There will be, then, no further 



