'"'' Level of no SStrainP — Claypole. 
85 
!2m 
But under so great a pressure as that which must exist at 
the depth indicated the formation of open spaces is impossi- 
ble. The material there existing, whatever its nature, must 
be i^lastic under a weight so enormous and at a temperature 
so high. Every space must be filled up almost before it is 
opened. The contraction in three dimensions is consequently 
reduced to shrinkage in one — the vertical — which becomes 
three times as great as it would otherwise be, the horizontal 
shrinkage in the other two directions entirely vanishing. This 
is the region of "stretching" of Mr. Davidson which might 
perhaps with greater propriety be called the region of "squeez- 
ing" when we consider the manner in which the forces act. 
In the annexed figure (1) the outer curve is intended to rep- 
resent the amount of contraction suffered by every thin shell 
of this crust between a depth of 400 miles and the surface. 
Beginning at the former with a slight 
curvature and very close to the radius to 
indicate small amount and slight increase, 
it gradually diverges as the cooling of 
the shells and consequently their con- 
traction increases until it reaches its 
maximum at the depth as already men- 
tioned of 70 or 80 miles. Returning 
from this point the curve rapidly ap- 
proaches the axis as the cooling and con- 
traction again diminish until at the sur- 
face where both become zero the two 
lines meet 
This curve then represents to the eye 
the contraction of every shell of the 
sphere' between the limits named and in- 
dicates a slow increase of the contraction 
in the lower layers, becoming more rapid as the maximum is 
approached and falling with greater rapidity, after this point 
is passed, to zei-o at the surface. Such is, without claiming 
too great accuracy, the actual condition of the region in ques- 
tion. 
Passing now to the second datum of the problem it is easy to 
see that owing to the contraction of ev^ery shell there must en- 
sue a regular and increasing descent of all except the lowest. 
Fig. 1. 
ABC-curve of shell con- 
traction. 
ADE— curve of loss of space 
by descent. 
B— level of greatest cooling. 
D — level of no strain. 
A— depth of 400 miles. 
