DIASTROPHISM AND THE FORMATIVE PROCESSES 485 



to be inherited from the ejection. The amount of this primitive 

 rotation, however, is not deducible directly from such rotations as 

 the bodies now have, for the present rotations are assignable chiefly 

 to the effects of planetesimal infall after the nuclei had become 

 planetary cores. Tliis later effect was conformable to a law of 

 equilibrium under which the rotations were sometimes accelerated 

 and sometimes retarded.^ 



When these inherited motions were strong enough to cause dis- 

 sipation, the nuclei of course vanished into planetesimals, but 

 when they were mild enough to be consistent with control, they 

 became vital factors in the process of concentration. The normal 

 system of thermal convection was gradually developed later and 

 hence had to conform to the inherited motions already in control 

 of the matter. In large nuclei the convectional motions might 

 come in time to dominate the inherited motions, but in the smaller 

 diffuse nuclei that were more quickly cooled it perhaps never came 

 to be more than a secondary factor. At any rate, the dependence 

 of the convective circulation on the inherited motions — merged 

 mainly into rotation later — must have given rise to a distinctly 

 gyratory system of circulation. This doubtless had some analogies 

 with the circulations of the atmosphere and of the ocean, which, 

 though essentially thermal, are profoundly affected by the earth's 

 rotation. A fundamental difference, however, needs notice. We 

 are here dealing with hot bodies whose radiation is primary. The 

 surface of a rotating body has its greatest convexity transverse to 

 the equator, while the polar surfaces are relatively flat. The 

 greatest radiation in proportion to the immediate submass there- 

 fore takes place in the equatorial region. In addition to this the 

 escape of molecules is aided by the centrifugal component of rota- 

 tion, which is greatest at the equator and sinks to zero at the poles. 



It has already been noted that escaping molecules carry off ther- 

 mal energy in relatively high amounts. The escape of molecules 

 may then be regarded as a form of quasi-radiation. It is, therefore, 

 a rather firm inference that the equatorial belt is the most effective 

 cooling tract of a hot, rotating body, though this may easily be 

 masked by the high radiation from all surfaces. 



' The Origin of the Earth (1916), p. 99. 



