Page:The evolution of worlds - Lowell.djvu/234

196 for example, owing to being cloudless and of a duller hue, turns out to have a computed mean temperature nearly equal to the Earth's,—a theoretic deduction which the aspect of the planet most obligingly corroborates. It thus enjoys a comparatively genial old age.

For what is specially instructive in planetary economy is that, on the whole, clear skies add more by what they let in than they subtract by what they let out. If the Earth had no clouds at all, its mean temperature would be higher than it is to-day. Thus as a planet ages a beneficent compensation is brought about, the Sun's heat increasing as its own gives out. Not that the foreign importation, however slight the duty levied on it by the air, ever fully makes up for the loss of the domestic article, but it tempers the refrigeration which inevitably occurs.

The subject of refrigeration leads us to one of the most puzzling and vexed problems of geology: how to account for the great Ice Age of which the manifest sign manuals both in Europe and in America have so intrigued man since he began to read the riddle of the rocks. Upon this, also, planetology throws some light.

If I needed an apology to the geologists for seeming again to trespass on their particular domain, I might refer to the astrocomico expositions put forward to account for the great Ice Age.

We can all remember Croll's "Climate and Time,"