Page:The evolution of worlds - Lowell.djvu/233

Rh temperature between sunset and sunrise, since during this interval the Earth receives no heat from the Sun. In the same way the efficacy of different atmospheric blankets may be judged. Thus the Earth parts with nine centigrade degrees' worth of its store on clear nights, and only four degrees' worth on cloudy ones, before morning. This is at sea-level. By going up a high mountain we get another set of depletions, and from this a relative scale for different atmospheric blankets. This is the principle, and we only have to fill out the skeleton of theory with appropriate numbers to find how warm the body is.

In doing so, we light on some interesting facts. Thus clouds reflect 72 per cent of the visible rays, letting through only 28 per cent of them. We feel chilly when a cloud passes over the Sun. On the other hand, slate reflects only 18 per cent of the visible rays, absorbing all the rest. This is why slate gets so much hotter in the Sun than chalk, and why men wear white in the tropics. White, indeed, is the best color to clothe one's self in the year around, except for the cold effect it has on the imagination, for it keeps one's own heat in as well as keeping the Sun's out. The modest, self-obliterating, white winter habit of the polar hares not only enables them to keep still and escape notice, but keeps them warm while they wait.

Astronomically, the effect is equally striking. Mars,