234 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



The problem of the earth as a body radiating its own heat into 

 space, however, is an example of a problem that may fail of treat- 

 ment as a unit problem by any one of the above-mentioned groups 

 acting alone. We are dealing not with a solid homogeneous spheroid 

 of uniform surface temperature radiating freely into space, but with 

 a rather heterogeneous body blanketed with several kinds of heat 

 insulators whose composition varies both with depth and with time. 

 Factors in the problem are: the production of heat by shrinkage; 

 the contributions of heat from radioactive sources; the earth's 

 present thermal conductivity and thermal gradient; the effect of 

 varying carbon dioxide, water, ozone, inorganic dust, and clouds, 

 upon the heat loss ; and the effects of the possibly very different at- 

 mospheres with which the earth has been blanketed in past ages. 

 The temperature at a given time and at a given distance from the 

 center, as for instance at the solid surface of the land, depends upon 

 a complex set of factors, and may well have been periodic in its 

 variations. 



The earth's volume and shape may have been similarly variable. 

 In addition to the variation of temperature, already mentioned, the 

 following are among the factors to be considered: the tides in the 

 solid earth (on which extensive experimental work has recently been 

 in progress) and the earth's properties as an elastic body; the vis- 

 cosity of the earth as a whole, with relation to long-continued forces, 

 and the existing state and method of maintenance of isostatic equi- 

 librium in its surface layer ; its breaking strength and form of rup- 

 ture under forces changing too rapidly to cause flow; and the lag 

 of elastic and viscous responses to changing forces, as in the case of 

 the addition or removal of continental ice sheets. 



Limitations of space forbid more than a sketchy outline of the 

 problems set before the Section of Geophysical-chemistry, but it is 

 hoped that the outline may have been sufficient to indicate the very 

 fundamental character of those problems. 



