ITS BALANCE OF GAIN AND LOSS. 121 



This is true of all districts lying near the 

 equator. In higher latitudes, a distinction be- 

 tween the seasons becomes in general the more 

 distinctly marked, the greater the difference be- 

 tween the longest and shortest days in the year. 

 As the length of the day increases, the maximum 

 of daily cooling of the ground diminishes; the 

 cause of heat increases in strength. The daily 

 maximum of heating now advances from day to 

 day. About the time of the longest day the 

 excess, of the influx of heat above the escape, is at 

 its greatest. Yet this point of time does not coin- 

 cide exactly with that of the greatest annual 

 income of heat, since the shortening of the day is, 

 for some time after the 21st of June, scarcely per- 

 ceptible. The daily gain in heat still continues to 

 exceed the loss, and it is not before the end of 

 July that the one effect is quite balanced by the 

 other. This then is the period of greatest annual 

 heat, at least in the temperate zone of the earth. 

 In August the excess of cooling is still scarcely 

 perceptible, but it is becoming more and more 

 apparent in the following months, till at last at the 

 shortest day, the diurnal loss of heat has reached 

 its maximum. The lowest temperature of the 

 year, however, does not occur till later, be- 

 cause for some time after the 21st of Decem- 

 ber the daily loss of heat exceeds the quantity 

 received. Thus it is that the coldest days of 



