12 Scientific Proceedings, Royal Dublin Society. 
tidal deforming action one third as much as if composed of water 
He has in view equilibrium tide deformation. But the centri- 
fugal deformation and the other are similar as regards the 
present matter, so that we can now argue from one to the other ; 
therefore the earth, though rigid as steel, will yield to the cen- 
trifugal force, or to any change of it, one third as much as if she 
were all water: so that only two-thirds of our difficulty remains 
to be removed. Dr. Croll has doubtless removed part of this 
remainder by the suggestion that the denuding agencies would 
tend to distribute detrital matter so that its surface would be 
everywhere nearly perpendicular to the direction of the result- 
ant of gravitation and the centrifugal force, and that in this 
way the shape of the earth would always nearly correspond 
to the actual amount of its diminishing centrifugal force. He 
also points out that the removal of material, from the equatorial 
towards the polar regions of the more slowly rotating earth would 
further help our cause by tending to delay the retardation of the 
earth’s rotation. Though the amount of relief thus afforded us is 
probably quite small, still we must not neglect it. 
Let us consider more closely this steel rigidity of our globe, 
which we do not presume to question, though the evidence 
for it,as Sir W. Thomson himself shews, now rests only upon 
the lunar fortnightly declinational, and the monthly elliptic, 
tides. There are different species of rigidity. Although our 
globe be practically as rigid and uncomplying as steel, relatively 
to the straining forces by which Sir William has tested her, she 
may nevertheless be as compliant as need be, relatively to the 
action with which we are now engaged. 
The peculiar character of viscous rigidity is well known. Per- 
haps the most familiar and most apt illustration of it is afforded 
by a stick of sealing-wax at an ordinary mean temperature. If 
we subject such a stick for a short time to a considerable trans- 
verse pressure, sufficiently below that which would cause frac- 
ture, no perceptible impression will be made thereon; and if 
that considerable pressure be a reciprocating one of short period, 
it may be continued for ever with as little effect. But if the 
same stick of sealing-wax be subjected to a very much smaller 
pressure, having always the same direction and continued for a 
very long time, it will give way thereto as a quite soft body 
