1320 
C'— A’ 
ft fee or = 0.000029. 
For the case of homogeneity this would become 
B= 0 000059. 
The value derived from the mean inclination of the moon’s 
equator was 
B = 0.000626 + .000002. 
Here again we find an enormous difference between the true 
values and the theory of CrLAIRAUT. 
8. The conclusion that the distribution of mass in the body of 
the moon is not in agreement with the theory of hydrostatic equi- 
librium, has already been reached by LaAPLAcg’). 
The mass constituting the crust of the earth is not in equilibrium 
either. But below the isostatie surface there is equilibrium. We 
are naturally led to assume that the depth of the isostatic surface 
is the depth at which the pressure of the outer layers becomes so 
large that tiie material of the earth behaves as a fluid and there- 
fore necessarily is in equilibrium’). To form an estimate of the 
pressure at the isostatic depth we can compute the pressure as it 
would be if the whole earth, including the crust, were in hydro- 
statie equilibrium. Then, treating the earth as a sphere, we have 
b 
Pp =| Lg dr, 
b—Z 
where y is the acceleration of gravity. Now 
__ fim ZT 
I= ee, i eA: 
Theretore 
b—Z 
For the earth the interval of integration is relatively small, and 
we can take A and D constant. Then D= D, and very approximately 
A=4D,. Further if Z= kb; we find 
pta Di ike et 
1) Mécanique Céleste, Livre V, Chapitre Il, § 18. 
2) So far as constant, or slowly varying forces and stresses are concerned. The 
behaviour of the material with respect to sudden forces is of no importance for 
our argument. 
