73 
The earth’s rigidity has been submitted to mathematical 
analysis by Sir W. Thomson. And he writes that this 
investigation “ suffices to disprove the hypothesis, hitherto so 
prevalent, that we live on a mere shell of solid substance, 
enclosing a fluid mass of melted rocks or metals ; and proves 
that the earth, as a whole, is much more rigid than any of the 
rocks which constitute its upper crust.” Thus a scientific 
doctrine, not long ago received as a certain truth, has been 
entirely reversed and set aside by the further progress of 
science. 
Another theory lately advanced is doomed, I suspect, to a 
similar fate. I mean the view first propounded, I think, by 
Mr. Croll, and adopted by Mr. Geikie in his Great Ice Age, 
and many others, that the supposed long ice-period of geo- 
logists can be explained by changes in the earth’s eccen- 
tricity. This would amount, by his calculation, to 10£ 
millions of miles, about 210,000 years ago. Now the pre- 
cession of the equinoxes, once in twenty thousand years, will 
place the winter solstice of the northern hemisphere in the 
aphelion. The combined effect of the two causes, when the 
winter half of the year was so much longer, exceeding the 
summer half more than twenty-six days, is thought enough to 
explain a long ice-period in the northern hemisphere. 
But in this hypothesis almost everything is precarious and 
uncertain. It is doubtful whether we can at all depend on the 
calculations of the past amount of the eccentricity. Elements 
wholly neglected might completely alter the reckoning for a 
time so long ago. The heating power of the sun, when one- 
fifth below the mean at the aphelion, would be one-fifth above 
it in the perihelion. The swiftness and the nearness exactly 
compensate each other; so that the amount of heat falling on 
the earth within one degree or minute of longitude is the 
same in every part of the orbit. Thus for the whole year the 
total heat which falls on the earth can be scarcely at all affected 
by the eccentricity, and even the ratio, for either hemisphere, 
of the total heat received in the summer and the winter half- 
year, from equinox to equinox, will mainly depend on the eccen- 
tricity, but on the inclination of the axis alone. While 
variations of the eccentricity could thus have only a slight 
and secondary effect in a period of many successive years, 
other causes might have a far greater effect, on which no exact 
data can be given, such as the proportions of land and sea, 
the varying transparency of the earth’s atmosphere, or changes 
in the absolute heating power of the sun. 
A. change of views once widely received is also in progress 
