CHAr. viiL] THE CAUSES OF GLACIAL EPOC[L'=!. 



159 



They have therefore been generally considered to be snow or 

 ice-caps, and to prove that Mars is now undergoing something 

 like a glacial period. It must always be remembered, however, 

 that we are very ignorant of the exact physical conditions of 

 the surface of Mars. It appears to have a cloudy atmosphere 

 like our own, but the gaseous composition of that atmosphere 

 may be different, and the clouds may be formed of other matter 

 besides aqueous vapour. Its much smaller mass and attractive 

 power must have an effect on the nature and extent of these 

 clouds, and the heat of the sun may consequently be modified 

 in a way quite different from anything that obtains upon our 

 earth. Bearing these difficulties and uncertainties in mind, let 

 us see what are the actual facts connected with the supposed 

 polar snows of Mars.^ 



Mars offers an excellent subject for comparison with the 

 Earth as regards this question, because its excentricity is now 

 a little greater than the maximum excentricity of the Earth 

 during the last million years, — (Mars excentricity 0.0931, 

 Earth excentricity, 850,000 years back, 0.0707) ; the inclina- 

 tion of its axis is also a little greater than ours (Mars 28° 51' , 

 Earth 23°, 27'), and both Mars and the Earth are so situated 

 that they now have the winter of their northern hemispheres 

 in perihelion, that of their southern hemisphere being in 

 aphelion. If, therefore, the physical condition of Mars were 

 the same or nearly the same as that of the Earth, all cir- 

 cumstances combine, according to Mr. Croll's hypothesis, to 

 produce a severe glacial epoch in its southern, with a perpetual 

 spring or summer in its northern, hemisphere ; while on the 

 hypothesis here advocated we should expect glaciation at both 

 poles. As a matter of fact Mars has two snow-caps, of nearly 

 equal magnitude at their maximum in winter, but varying very 

 unequally. The northern cap varies slowly and little, the 

 southern varies rapidly and largely. 



1 The astronomical facts connected with the motions and appearance of 

 the planet are taken from a paper by Mr. Edward Carpenter, M.A., in the 

 Geological Magazine of March, 1877, entitled, "Evidence afforded by 

 Mars on the subject of Glacial Periods," but I arrive at somewhat different 

 conclusions from those of the writer of the paper. 



