REPLY TO CRITICS. 35 



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 transparency 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 nio-ht, and also durino- 

 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. 



It is true that the land is hotter during the day, and 

 also during the summer, than the ocean, but it is found 

 that the more equable temperature of the ocean gives 

 a higher mean. This is further shown from another 

 consideration. The land is more indebted for heat to 

 the ocean than the ocean is for heat to the land. For 

 example, a very considerable portion of the warmth 

 enjoyed by north-western Europe is derived from the 

 Atlantic. In like manner, western America is indebted 

 to the Pacific for a large amount of its heat. In 

 addition, an immense quantity of the heat received 



