88 ROUGH WAYS MADE SMOOTH. 



in the right who have maintained in recent times the theory 

 that in the case of a cooling globe, such as the earth or 

 moon at the stage just described, the crust would in the 

 first place contract more quickly than the nucleus, while 

 later the nucleus would contract more quickly than the crust. 

 This amounts, in fact, to little more than the assertion that 

 the process of heat radiation from the surface would be more 

 rapid, and so last a shorter time than the process of con- 

 duction by which in the main the nucleus would part with its 

 heat. The crust would part rapidly with its heat, contracting 

 upon the nucleus ; but the very rapidity (relative) of the pro- 

 cess, by completing at an early stage the radiation of the 

 greater portion of the heat originally belonging to the crust, 

 would cau-e the subsequent radiation to be comparatively 

 slow, while the conduction of heat from the nucleus to the 

 crust would take place more rapidly, not only relatively but 

 actually. 



Now it is clear that the results accruing during the two 

 stages into which we thus divide the cooling of the lunar 

 globe would be markedly different. During the first stage 

 forces of tension (tangential) would be called to play in the 

 lunar crust ; during the later stage the forces would be those 

 of pressure. 



Taking the earlier stage, during which the forces would 

 be tensional, let us consider in what way these forces would 

 operate. 



At the beginning, when the crust would be comparatively 

 thin, I conceive chat the more general result of the rapid 

 contraction of the crust would be the division of the crust 

 into segments, by the formation of numerous fissures due to 

 the lateral contraction of the thin crust. The molten mat- 

 ter in these fissures would film over rapidly, however, and 

 all the time the crust would be growing thicker and thicker, 

 until at length the formation of distinct segments would no 

 longer be possible. The thickening crust, plastic in its lower 

 strata, would now resist more effectively the tangential ten- 

 sions, and when yielding would yield in a different manner. 



