Physical Theories of the Earth— Refide. 107 
numbers of the American Geologist, Prof. E. W. Claypole, haH 
discussed from a geological point of view the eft'ecfc of the dis- 
covery of a "level of no strain" in a cooling globe on current 
geological thought. The article is fairly and dispassionately 
written and evinces an appreciative knowledge of these later 
physicial investigations. There are, however, some underlying 
inferences with which I cannot agree and perhaps as being the 
first to demonstrate that there exists a stratum, or more ac- 
curately speaking a "level," in a cooling globe in which neither 
extension nor compression takes place, I may be allowed to 
discuss some of the points Prof. Claypole touches upon. 
My views on the strains set up in a cooling globe were first 
published iu chapter xi, "Origin of Mountain Ranges" 1886, 
though I had arrived at a clear conception of the existence of a 
neutral zone or shell five years previously, as can be conclu- 
sively proved by my notes. Since then, as described by Prof. 
Claypole, investigations have been carried on by Davison, Fisher 
and G. H. Darwin with the effect of fixing the limits of depth 
at which this neutral shell or "level of no strain" must occur 
in the case of our own globe. 
And here [ may remark — what appears to be lost sight of by 
these investigators — that the existence of such a level of-no- 
strain is quite independent of the general rigidity of the globe 
which is another question. In fact I was led to the discovery 
by considering the probable behavior of such a crust as is as- 
sumed by the supporters of the contraction hypothesis to exist 
in the case of our own globe. It matters not whether the 
nucleus be solid, fluid, or plastic; for so soon as the exterioi- 
shell becomes solid by cooling, A\*ithin that shell there will 
exist a level-of-no-strain; always provided that the continuity 
of the shell is preserved by pressure produced by gravitation. 
This may not at first sight be obvious but it arises from the 
relations between the circumferential and radial contractions 
of such a cooling globe, by which the thickness of the solidify- 
ing crust is always greater than the depth of the level-of-no- 
strain. 
Professor Claypole while giving due weight to these investi- 
gations thinks that the mathematicians may have erred in 
their numerical calculations, and placed the neutral shell in 
the case of our own globe too near the surface. His ground 
