THE CAUSE OF THE GLACIAL PERIOD. 477 



As already intimated, the weak point in Mr. Croll's the- 

 ory is the general state of uncertainty as to the laws regulat- 

 ing the absorption, retention, and distribution of the sun's 

 heat upon the earth. It is evident that the heat upon which 

 the earth is dependent is that of the sun ; since, as Professor 

 Newcomb has shown, the total amount of heat received from 

 the stars is probably not one-millionth part of that received 

 from the sun.* Now, as all admit that the annual amount 

 of heat received, from the sun is not affected by changes 

 either in the eccentricity of the earth's orbit, or in the rela- 

 tion of the poles to that eccentricity, it is only the question 

 of the retention and distribution of heat with which we have 

 to do. And here we come to a most obscure realm of sci- 

 entific investigation, where ignorance is still profound. The 

 reason why the summit of a mountain is cold is not because 

 of lack of heat from the sun, but it arises rather from the 

 facility with which the heat is dissipated by radiation. On 

 the contrary, the reason why the atmosphere of a greenhouse 

 is warmer than that upon the outside is not because it receives 

 more heat, but because it retains more. The intenser heat- 

 rays of the sun readily penetrate the glass cover, while the 

 less intense rays of radiated heat from the earth are unable 

 to do so in return. It is well known, also, that clouds pre- 

 vent a frost by checking the radiation from the surface of the 

 earth. The laws regulating the influence of the atmosphere 

 and the floating particles contained in it, over the retention 

 of the sun's heat in its lower strata, are as yet but little un- 

 derstood. There is here an almost unlimited field for inves- 

 tigation and discovery. 



And this, as just remarked, is the weak point of Mr. 

 Croll's theory. Everything here depends upon the forces 

 which distribute the heat and moisture over the land-sur- 

 faces. It is by no means certain that, when the winters of 

 the northern hemisphere occur in aphelion, they will be 

 colder than now. Whether they would be so or not depends 



* "American Journal of Science," vol. cxi, 1876, p. 264; vol. cxxvii, 1884, 

 p. 22. 



