09 O 



Mi*. J. Croll on the supposed greater Loss of Heat 



will therefore be the rate at which it will lose that heat by radia- 

 tion. The total quantity of heat received during- winter by the 

 southern hemisphere is exactly equal to that received during 

 winter by the northern. But as the southern winter is longer 

 than the northern, the rate at which the heat is received during 

 that season must be less on the southern hemisphere than on 

 the northern. Now this less rate, were it not for a circum- 

 stance presently to be noticed, ought exactly to compensate 

 for the longer winter. The southern hemisphere loses heat 

 during a longer period than the northern • but then it does not 

 lose it so rapidly. Therefore the total quantity of heat lost, were 

 it not for the circumstance alluded to, would be the same on both 

 hemispheres. The same mode of reasoning is equally applicable 

 to the summers of the two hemispheres. The southern summer 

 is shorter than the northern ; but the heat is more intense, and 

 the surface of the ground kept at a higher temperature ; conse- 

 quently the rate of radiation into space is greater. 



When the rate at which a body receives heat is increased, the 

 temperature of the body rises till the rate of radiation equals the 

 rate of absorption, after which equilibrium is restored ; and when 

 the rate of absorption is diminished, the temperature falls till 

 the rate of radiation is brought to equal that of absorption. 



But notwithstanding all this, owing to the slow conductivity 

 of the ground for heat, more heat will pass into it during the 

 longer summer of aphelion than during the shorter one of peri- 

 helion ; for the amount of heat which passes into the ground 

 depends on the length of time during which the earth is receiving 

 heat, as well as upon the amount received. Also in like manner 

 during the longer winter in. aphelion, more heat will pass out of 

 the ground than during the shorter one in perihelion. Suppose 

 the length of the days on the one hemisphere (say the northern) 

 to be 23 hours, and the length of the nights, say, 1 hour; while 

 on the other hemisphere the days are 1 hour and the nights 23 

 hours. Suppose also that the quantity of heat received from the 

 sun by the southern hemisphere during the day of 1 hour to be 

 equal to that received by the northern hemisphere during the 

 day of 23 hours. It is evident that although the surface of the 

 ground on the southern hemisphere would receive as much heat 

 from the sun during the short day of 1 hour as the surface of 

 the northern hemisphere during the long day of 23 hours, yet, 

 owing to the slow conductivity of. the surface for heat, the amount 

 absorbed by the ground would hot be nearly so much on the south- 

 ern hemisphere as on the northern. The temperature of the 

 surface during the day, it is true, would be far higher on the 

 southern hemisphere than on the northern, and consequently 

 the rate at which the heat would pass into the ground would be 



