312 The Past History of our Moon. (July, 
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 contra¢t 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, andso lastashortertime than the process of conduétion | 
by which in the main the nucleus would part with its heat. 
The crust would part rapidly with its heat, contra¢ting upon 
the nucleus ; but the very rapidity (relative) of the process, 
by completing at an early stage the radiation of the greater 
portion of the heat originally belonging to the crust, would 
cause 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 that 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 distin¢ét 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. 
At this stage, in all probability, it was that processes such 
as those illustrated by Nasmyth’s globe experiments took 
place, and that from time to time the crust yielded at parti- 
cular points, which became the centres of systems of radiating 
fissures. Before proceeding, however, to consider the results 
of such processes, let it be noted that we have seen reason 
to believe that among the very earliest lunar formations 
would be rifts breaking the ancient surface of the lunar crust. 
I distinguish in this way the ancient surface from portions 
of surface whereof I shall presently have to speak as formed 
at a later time. 
