ORIGIN AND DIFFERENTIATION 543 



substantially the same zonal action on the outside of this cloud spheroid 

 that it noAv does upon the present cloud surface and, within the limits of 

 certain modifying factors, upon the earth's surface. To quote from 

 Hilgard,^' who has written in strong advocacy of this hypothesis : 



"The tropical bolt with its strong ascending currents, low barometer, and 

 high temperatures ; two adjoining arid belts with descending currents and 

 high barometer, and the temperature zones to poleward, with variable but 

 generally low barometer, would be defined on the cloud spheroid as they now 

 are on the earth's surface, but with greater regularity, though perhaps less 

 sharply defined.*" 



A certain amount of earth heat, carried up by convection currents, is 

 lost by radiation into space, bnt the solar radiation falling on the outside 

 of the cloud spheroid would in part compensate for this loss, and thus 

 the sun would actually become a conservator of earth heat without 

 directly influencing the temperature of the earth's surface. This great 

 amount of moisture in the atmosphere naturally led to greater and more 

 rapid precipitation, and the rainfall was imdoubtedly greater than at 

 present. 



The further elaboration of Manson's hypothesis has been so graphically 

 and succinctly stated by Hilgard that I venture to quote from his paper. 

 He says : 



"The isothermal spheroids or shells corresponding to our present tempera- 

 tures would at first be at heights more considerable than at present ; but as 

 the heat carried up from the earth's surface was more and more lost by radia- 

 tion into space from the exterior cloud-surface, the isothermal shells would 

 gradually descend, and the temperature of the falling rains would become 

 lower, so as under favorable conditions to fall as snow. It is clear that snow- 

 fall might occur at any period of the earth's evolution on high mountain 

 ranges or plateaus, and there the accumulations of snow might at any period 

 have formed nevees and glaciers with their well-known effects. . . . Owing 

 to the higher radiating power of the earth-surface as compared with the ocean, 

 as well as its much lower specific heat, the earth must have cooled more rap- 

 idly than the oceans by radiation alone. In addition to this, the water flowing 

 from it into the seas would carry off a large amount of heat. Even while the 

 ocean still received heat from its bed, the land areas would be a cooling agency 

 especially for the ocean depths, while the warm oceanic surface waters would 

 be supplying abundant vapor for precipitation on the relatively cooler land 

 areas. The latter would finally fall to so low a temperature as to receive 

 their precipitation in the form of snow, thus inaugurating the glacial period, 

 during which the isothermal shell of, say, the freezing point of water, and 

 below, descended near to the earth's surface. As the ocean also gradually 

 cooled and evaporation diminished, the protecting cloud-envelope became 

 thinner, first in the tropics and its flanking belts of lesser rainfall (which 



27 E. W. Hilgard. 



