14 . Scientific Proceedings, Royal Dublin Society. 
the strong compression keeps the body still apparently quite 
rigid when tested by short-period straining ferces. But as the 
two rigidities depend upon very different conditions, they must 
be, in all probability, very different in kind. The former 
depended alone on the substantive cohesion of the particles and 
the (interfering) heat oscillations; the latter on the new strength 
of cohesion, to which the compression gives rise, and to the far 
more violent heat oscillations. 
The compression has probably but little direct effect in pro- 
ducing rigidity. Its principal effect is to enable the attraction of 
cohesion to have place by the molecules being held together 
closely enough ; the cohesive attraction now depends for its oppor- 
tunity of acting on the pressure. But that pressure cannot be 
absolutely uniform among the groups of molecules; it is so 
statistically only (somewhat as in a gas, although the cases are 
very different) ; this must be so at any rate, and especially under 
the interfering jostling action of the excessively violent oscil- 
lations; and therefore the cohesion among the molecules, or 
groups of molecules, cannot be uniform; and if the pressure do 
not prevail too much over the thermal excitement the variation 
of cohesion may well be sufficient to make some of the groups of 
molecules at any instant, and all, ora sufficient proportion of them 
in turn, unstable under certain amounts of strain—and these are 
precisely the supposed molecular conditions of a viscous body. 
Now when we consider that the mean density of the whole 
alobe is only about double that of its superficial parts, and even 
that that small increase of density is doubtless partly due to the 
higher specific gravity of the interior parts, and therefore only 
partly to the compression, we may conclude that the internal 
high temperature is largely able to hold its own, as to its loosening 
tendency, against the increased cohesion resulting from the con- 
densation under the internal pressure. Sir William himself con- 
templates all the interior parts of the earth being somewhat near 
the melting points corresponding to the actual pressures. There- 
fore the great probability seems to be that the rigidity of our 
globe, though as high in degree as supposed by Sir. W. Thom- 
son, is, as to species, a viscous rigidity, which will answer our 
purpose quite well. That of the crust is not worth mentioning. 
But now we come to what seems to be a satisfactory confirm- 
ation of this position. I was myself the more struck by it 
