S. Newcomb—Crolts Climate ayid Time. 269 



tent diatbermanous to radit 

 absorbs the ra3^s from water a 

 cording to the usually received laws of heat, one body can be 

 heated from another only when the latter is the warmer of the 

 two, and the rapidity with which the heating process goes on 

 depends on the difference of temperature, no matter whether 

 the heat passes by conduction, or by radiation. If, then, the 

 air is really heated by contact with the ground more rapidly 

 than by contact with the ocean, it can only be because the 

 ground is hotter than the ocean, which is directly contrary to 

 the theory Mr. Croll is maintaining. The statement that ' the 

 aqueous vapor of the air is diathermanous to radiation from 

 land, but not to that from water, is quite new to ns, and very 

 surprising ; but if it be true, Mr. Croll assigns directly contrary 

 effects to the same cause in (1) and (2). Reasoning as in (1), 

 he would have said that the air over the land, owing to its 



tos 



being transparent, can acquire heat from the ocean only by the 



slow process of convection. 



(3.) "The air radiates back a considerable portion of its heat, 

 and the ocean absorbs this radiation from the air more readily 

 than the ground does." Here we have the air giving back to 

 the ocean the same heat which it absorbs from it, and thus 

 heating it. Apparently, Mr. Croll thinks that air and ocean 

 can thus alternately heat each other up to an indefinite extent, 

 by natural radiation, without any necessity for more than a 

 mere nest-egg of heat to start with from any outside source. 

 (4) seems to be little more than a repetition of (2) in a different 

 form. 



Another idea of the author which calls for explanation is 

 that solar heat absorbed by the atmosphere is entirely lost, so 

 far as warming any regi(m of the globe is concerned. For 

 instance, in comparing the relative amount of heat received 

 from the sun by the equatorial and the arctic regions, he thinks 

 It a mistake not to allow for the fact that a greater percentage 

 of the heat is absorbed by the atmosphere in the polar regions 

 than at the equator. From the care he takes to subtract this 

 percentage from the amount of heat received by the polar 

 regions, he seems to think that the heat thus absorbed is totally 

 lost, and does not warm the atmosphere at all. But a moment's 

 reflection must show that as all this absorption must occur 

 within three or four miles of the earth's surface, and probably 

 naif of it within a single mile, or two miles at most, while the 

 arctic regions are more than 2,000 miles in diameter, it makes 

 no difference what portion of the heat is absorbed by the atmos- 

 phere. In the one case, the atmosphere is warmed directly bj 

 Am. .Jopr. Sci.-Third Series, Vol. XI, No. 64.-Apbil, 1876. 



