30 DISCUSSIONS IN CLIMATOLOGY. 



as in (1), he, Mr. Croll, would have said that the air 

 over the land, owing to its transparency for the heat- 

 rays from the land, becomes heated to a greater height 

 rapidly, while the air over the ocean, not being trans- 

 parent, can acquire heat from the ocean only by the 

 slow process of convection." I would have said no 

 such thing. Radiation from the surface of the land 

 will, no doubt, penetrate more freely through the 

 aqueous vapour than radiation from the ocean ; but 

 the aqueous vapour will not absorb the radiation of 

 the land so rapidly as that of the ocean, for the ocean 

 gives off that quality of rays which aqueous vapour 

 absorbs most rapidly. 



This is not in opposition to what I have stated in 

 reason (1) ; for, if the ground were transparent to the 

 sun's rays like water, evidently the total quantity of 

 heat absorbed by it would be greater than that by the 

 ocean. But radiation from the sun heats only the 

 surface of the ground; all below the surface depends 

 for its supply on the slow process of conduction, 

 whereas the ocean is heated by direct radiation to 

 great depths. Consequently the total quantity of 

 heat absorbed by the ocean, say per square mile, in a 

 given time, is greater than that absorbed by the land. 



Third. — ' The air radiates back a considerable por- 

 tion of its heat, and the ocean absorbs this radiation 

 from the air more readily than the ground does. The 

 ocean will not reflect the heat from the aqueous vapour 

 of the air, but absorbs it, while the ground does the 

 opposite. Radiation from the air, therefore, tends 

 more readily to heat the ocean than it does the land.' 



" Here we have," he says, " the air giving back to 

 the ocean the same heat which it absorbs from it, 

 and thus heating it." If Professor Newcomb means 

 by this same heat the same amount of heat, then I 



