30 The Interior of the Earth — Claypole. 
and may be neglected, at the depth of about 400 miles. From 
this level it increases upward to a maximum which he places at 
a depth of seventy-two miles and then diminishes again to an 
imperceptible amount at the surface. 
Of course the couche of greatest contraction closely accom- 
panies that of greatest cooling — if indeed the two are not 
identical — and this accordingly is found by Mr. Davison at 
about the same depth — 72 miles, the contraction diminishing as 
the cooling, both upward and downward. 
Piu-suing his investigations Mr. Davison shows, as Mr. T. 
M. Reade had shown a few months previousl}-, that between 
this layer of greatest contraction, where the space exceeds the 
matter, and the surface, where the matter exceeds the space (the 
latter being zero), must lie the layer of "no strain" and this he 
places, as said above, at the depth of about five miles. Below 
this the contraction from cooling exceeds the diminution of space 
by descent and the layers are consequently squeezed out or 
flattened to fill the vacancy. Above this the strata are crumpled 
or crushed because they are too large for the smaller space into 
^vhich they are sinking. 
The Rev. O. Fisher of Cambridge, England, in a review of 
the papers above quoted obtains results which differ considera- 
bly from those above given. Assuming the temperature of 
solidificacion at 7000 degrees F., he finds the depth of the shell 
of greatest cooling and contraction at fifty-four miles, and that 
of the level of "no strain" at two miles; while assuming an 
initial temperature of 4000 degrees F., the former would lie at 
a depth of thirty-one miles and the latter at a depth of 0.7 of a 
mile. These discordant results show us that geology is not yet 
in a state to speak with confidence upon the exact condition of 
the interior of the earth at given but inaccessible depths. In 
the former case the result would be attained in ninety-eight 
millions and in the latter in thirty-three millions of years. 
Mr. Davison also remarks on the indisputable fact that in a 
cooling globe this layer of "no strain" which was once at the 
surface is constantly descending with time and he states that 
*' within certain limits its depth increases with the square root 
of the time;" whereas according to Mr. Fisher's calculation it 
varies as the time. Its descent in the latter case must be much 
more rapid than in the former. 
