"- Level of no Strain.^ — Claypole. 87 
Both diverge upward until they reach the shell of maximum 
cooling. Bat above this level the former returns toward the 
axis as contraction diminishes while the latter continues to di- 
verge to the surface. It follows therefore of necessity that at 
some point these curves must intersect each other. This means 
that at the point where this intersection occurs the values 
which they represent are equal, or that the contraction in con- 
sequence of the cooling of the shell occupying that level exact- 
tly equals the loss of space to which it is compelled to submit 
on account of its descent to a position nearer to the centre of 
the sphere. 
This is "the level of no strain" of the authors named at the 
beginning of this paper. By this term Messrs. Read and 
Davison, to whom the conception is due, imply that the 
layer in the Earth's crust occupying this position is neither 
squeezed out to fill nascent empty spaces as in the lower part 
of the 400-mile shell, nor crushed by compression owing to 
the descent of layers already cool and therefore incapable of 
much further contraction to a level in the sphere too narrow 
to receive them without folding or crushing. It is a peaceful 
zone in a crust constantly troubled, a resting layer in an ever 
restless mass. 
In the two papers already alluded to published in this maga- 
zine in June and July, 1888, I pointed out sundry geological 
reasons for suspecting that the "level of no strain" had been 
set too high by its discoverers and investiga- 
tors. These need not be repeated here but it 
may not be uninteresting in conclusion to 
point out a few of the changes through which 
this zone bas passed since the consolidation of 
the Earth took place. As the authors have re- 
marked it has descend"ed with time to. deeper 
and deeper positions. For example at the mo- 
ment of consolidation no such layer existed. 
The temperature increased from the surface 
Fig- 2. down perhaps to the centre though not at a 
constant rate. The superficial shell was that of greatest 
cooling and contraction. The two curves described in this 
paper were then as shown in fig. 2 and both reached zero at a 
small depth. But after the consolidation had taken place and 
the superficial layer had grown cooler but still was cooling 
