Professor Newcomb's " Rejoinder" 279 



(' Climate and Time/ and elsewhere) that A, the water hemi- 

 sphere, ought to have a higher mean temperature than B, the 

 land hemisphere, because the former has a greater difficulty 

 in getting quit of the heat which it is receiving from the sun 

 than the latter. The question then arises, How is it that the 

 water hemisphere has a greater difficulty than the land hemi- 

 sphere in getting rid of its heat ? It is mainly due to that 

 cause which Prof. Newcomb says is quite new to him, viz. the 

 fact that the aqueous vapour of the air is far less diathermanous 

 to radiation from water than from land. It is a curious fact 

 that Prof. Newcomb, in his " Rejoinder," entirely overlooks 

 this cause assigned by me, although I have stated it fully in 

 my fourth reason. The period of the heat-vibrations of the 

 aqueous vapour of the air is the same as that of the ocean, and 

 consequently the aqueous vapour will absorb radiation from 

 the ocean more readily than from the land. A considerable 

 portion of the heat absorbed by the aqueous vapour of the air 

 is thrown back upon the ocean, and in this way the aqueous 

 vapour acts as a screen, or like the glass of a greenhouse, in 

 preventing the ocean from getting quit of its heat so rapidly 

 as the land. The result is that the temperature of equilibrium 

 of the ocean must be higher than that of the land. In other 

 words, before the ocean can manage to throw off its heat into 

 space as rapidly as it is receiving it, its temperature must be 

 higher than that of the land. 



The foregoing conclusion follows so obviously from the 

 known properties of aqueous vapour and the principles of 

 thermodynamics that I can hardly believe Prof. Newcomb 

 will call it in question. But he will ask how can the trans- 

 parency of the ocean for heat-rays, the mobility of its particles, 

 and the greater store of heat which it possesses, be a reason 

 why its mean temperature should be higher than that of the 

 land? I thought I had made all this clear. The reason 

 becomes apparent when we consider why it is that the surface 

 of the ocean during night and also during winter is warmer 

 than the surface of the land. The ocean in temperate regions 

 seldom sinks to the freezing-point, while the land is frequently 

 frozen for months. The cause is obvious enough : at night, 

 when the surface of the ocean becomes cool, the cold particles 

 sink and their places are supplied by warm particles from 

 below, and so long as the heat stored up remains the surface 

 can never become cold, Were it not for the transparency of 

 water for heat-rays, it would be impossible that the ocean 

 could obtain a supply of heat sufficient to maintain its surface- 

 temperature during the entire winter; and, on the other hand, 

 were the particles not mobile, this store could be of little service. 



