CHAP. VIII THE CAUSES OF GLACIAL EPOCHS 165 



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 

 00031, Earth excentricity, 850,000 years back, 0-0707); 

 the inclination 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 pcrilielion, that of their southern 

 hemisphere being in afJidion. If, therefore, the physical 

 condition of Mars were the same or nearly the same as that 

 of the Earth, all circumstances combine, according to Dr. 

 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. 



In the year 1830 iha so^tthern ^xio^\ was observed, during 

 the midsummer of Mars, to diminish to half its former 

 diameter in a fortnight (the duration of such phenomena 

 on Mars being reckoned in Martian months equivalent to 

 one-twelfth of a Martian year). Thus on June 23rd it 

 was 11° 30' in diameter, and on July 9th had diminished 

 to 5° 46', after which it rapidly increased again. In 1837 

 the same cap was observed near its maximum in winter, 

 and was found to be about 35° in diameter. 



In the same year the northern snow-caj^ was observed 

 during its summer, and was found to vary as follows : — 



We thus see that Mars has two permanent snow-caps, of 

 nearly equal size in winter but diminishing very unequallv 



