96 The Ainerio(in Geologist. An'^'ust, isgri. 
The relative distances of tlie earth and Mars from the sun 
being as 1 : 1.5, the light and heat that the_y receive is in pro- 
portion to 9: 4. Consequently the sunlight and sun heat on 
Mars are a little more than half as strong as that we feel. 
Yet in spite of this diminution his polar regions are kept above 
the freezing point throughout the summer and his polar ice 
nearly or altogether disa2:)pears. The excessive tenuity of his 
atmosphere, rendering cloud and fog almost impossible, may 
combine with the scarcity of water above mentioned to pro- 
duce this result. 
The aqueous vapor of which his atmosphere may in great 
part consist is a third factor in the problem. By retaining 
near the surface of the planet the reflected solar heat the tem- 
perature of the lower laj^ers must be considerably raised by 
day and their cooling by radiation equally reduced at night. 
The combined result is apparently to render the Martian cli- 
mate far from intolerable during the summer, and the contin- 
ued flow of vapor and its condensation must to some degree 
at least mitigate the otherwise intense severity of the long 
twelve-month winter. 
In considering the glacial relations of Mars it must be fur- 
ther remembered that the eccentricity of his orbit far exceeds 
that of our own earth at present or that which it has attained 
at any known epoch in the past. These two amounts are rep- 
resented by the tlgures 0.01617 and 0.09326 respectively. The 
latter is nearly six times as great as the former. The axes of 
the orbits and the perihelion and aphelion distances of the 
planets also greatly vary. At the southern summer solstice 
the earth is about three million miles nearer to the sun than 
in midwinter. But Mars at the same season is twenty-eight 
million miles nearer than at his southern winter solstice. One 
consequence of this is the lengthening of the latter season. 
On the earth at the present time the southern summer is eight 
days shorter than the winter, but on Mars the difference in the 
same direction amounts to 74 days, the planet spending 306 
days in the perihelial and 380 days in the aphelial division of 
its orbit. If the eccentricity were the main cause of glacial 
climates Mars must then be in a very favorable condition for 
glaciation. But as it is possible that his eccentricity may 
yet reach the yet higher figure of 0.14224 these conditions are 
