226 Mr. J. Croll on the supposed greater Loss of Heat 



become very sensibly affected by the low temperature of the sur- 

 face the frozen soil would be thawed. 



The storing up of heat or cold in the ground has in reality 

 very little to do with climate. Some physicists explain, for ex- 

 ample, why the month of July is warmer than June by referring 

 it to the fact that by the month of July the ground has become 

 possessed of a larger accumulation of heat than it possessed in 

 June. This explanation is evidently erroneous. The ground in 

 July certainly possesses a greater store of heat than it did in 

 June; but this is not the reason why the former month is hotter 

 than the latter. July is hotter than June because the air (not 

 the ground) has become possessed of a larger store of heat than 

 it had in June. And why the air is warmer in July than in 

 June is this : it is with extreme difficulty that the air can be- 

 come heated by the direct rays of the sun ; it is by means of 

 contact with the hot surface of the ground and by radiation 

 from the earth that the air becomes slowly heated. Conse- 

 quently, although the sun^s heat is greater in June than it is in 

 July, it is near the middle of July before the air becomes pos- 

 sessed of its maximum store of heat. We therefore say that 

 July is hotter than June because the air is hotter in the former 

 month than in the latter, and consequently the temperature in 

 the shade is greater in the former month than in the latter. 



If the distribution of sea and land were the same in both 

 hemispheres, it follows, according to theory, that, owing to the 

 excess of 184 hours in the length of the southern polar winter 

 over the northern, there would be a very slight loss of heat 

 on the southern hemisphere and a very slight gain of heat on 

 the northern. But owing to the present distribution of sea and 

 land, the very reverse in reality takes place. At present the 

 northern hemisphere loses by radiation far more heat than the 

 southern. The reason of this is obvious. The greater part of 

 the southern hemisphere is occupied by sea. Water is a much 

 worse radiator than land. There are a great many reasons for 

 this, a few of which may be enumerated : — (1) The temperature of 

 the surface of the water does not rise so high under the direct rays 

 of the sun as that of the surface of the ground. (2) The heat-rays 

 from the sun penetrate the water to a considerable depth, and in 

 this case it is only a part of the heat that is received by the surface 

 of the water, whereas in regard to land all the heat is received 

 by the surface. The temperature of the surface of the land is 

 thus raised enormously, and the heat rapidly thrown back into 

 stellar space ; this effect is also increased by the fact that the 

 specific heat of the land is not one-half that of water. (3) The 

 ground can only store up heat by the very slow process of con- 

 duction, whereas water, by the mobility of its particles and 



