1321 
The material out of which the meon is built up is probably not 
very different from that of the outer layers of the earth. We will 
therefore assume that it requires the same pressure to be fluid 
enough for the state of permanent equilibrium. If now on the moon 
the depth of the isostatic surface, if there be one, is Z” = k/b’, we have 
vo 
Now we can put A’. D’ = aD,’?. If the moon were homogeneous, 
we should have «= 1. If the density increases towards the centre, 
then at the outer surface a <1, and at the centre a > 1. If «, be 
a certain mean value of « over the interval of integration, we have 
p = tra Db [Eh] 
Now 
b=0272b , D,'= 0.610 D,. 
Taking further 4—= 0.018, we find from the condition p’ = p 
0.32 
Ft, 
a, 
If we take a, = 1, we find 
k= 0.40. 
Most probably the true value of @, does not differ much from 
unity. The isostatic surface in the moon would thus be situated at 
a depth of about two fifths of the radius, and little more than one 
fifth of the total volume would be inclosed within it. Of course 
there can be no question of an isostatic compensation as there is 
in the earth. The differences of the moments of inertia are almost 
entirely determined by the irregularities in the “crust”, which here 
contains by far the largest part of the mass, and the small central 
part has only very little influence. 
This reasoning, of course, is not entirely rigorous, but it undoubt- 
edly points out the true reason why the theory of Crairavur, which 
in the case of the earth agrees so well with the actual facts, is not 
at all applicable to the moon. 
