24: THE POLAK GLACIERS. 



It is a peculiar property of bodies revolving in elliptical orbits, 

 that they travel faster when near the center of attraction than 

 when further away. It follows, from the second of the three 

 great laws of planetary motion discovered by Kepler, that the 

 line connecting the two bodies must pass over equal areas in equal 

 times. The earth passes through our winter portion of its orbit, 

 that is, from autumnal to vernal equinox, in eight days less time 

 than through the summer part of it. In the southern hemi- 

 sphere of course the condition of things is reversed, and the 

 winter there is eight days longer than the summer. Moreover, 

 the sun is at its greatest distance from the earth during the long 

 southern winter, and at its least in the short northern winter. 



Of the two causes, I regard the first as of main importance. 

 Distance from the sun, whatever theory may be, does not seem 

 to have much effect upon climate. The southern summers, when 

 the sun is over 3,000,000 miles nearer the earth, are said to be 

 even some degrees cooler than the same seasons in corresponding 

 localities of the northern hemisphere. And to take an extreme 

 example, Mars, which is 50,000,000 miles further from the sun 

 than the earth is, has snow-lines about its poles which reach no 

 nearer the equator than on our planet in corresponding seasons. 

 But the excess or diminution of eight days in the winters of 

 climates which, even in their warmest seasons, barely balance on 

 the thawing point of ice, is a true cause in polar conditions and 

 differences. Considering that these days affect chiefly the period 

 of briefest sunshine, it amounts to quite one-twentieth of the 

 whole power of the sun on a hemisphere. This difference would 

 not be apparent in the warm regions of the globe, where there is 

 always an excess of heat which is carried off by evaporation and 

 ocean-currents ; but it would exert nearly its full force in polar 

 regions which are unaffected by those influences. 



It cannot be denied that it is the sun's heat which prevents the 

 temperature of the earth from sinking to, or very near to, the 

 absolute zero of cold, wherever in the thermometrical scale that 

 may be. Chemists have produced a cold estimated at 257 below 

 zero, of Fahr.* It is not by any means probable that this reaches 



* The temperature of stellar space is estimated by Sir John Herschel and others at -239 Falir. 



