160 



ISLAND LIFE. 



[part I. 



In the year 1830 the southern snow 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 eqnivalent 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-cap was observed during 

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



May 41 h. Diameter of spot 31° 24 



Jimo 4tb. „ „ 28' 0' 



„ 17th. „ „ 22° 54' 



July 4th. „ „ 18° 24" 



„ 12th. „ „ 15° 20' 



„ 20th. „ „ 18= 0' 



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

 equal size in winter but diminishing very unequally in summer, 

 when the southern cap is reduced to nearly one-third the size 

 of the northern ; and this fact is held by Mr. Carpenter, as it 

 was by the late Mr. Belt, to be opposed to the view of the 

 hemisphere which has winter in ajphelion (as the southern now 

 ha's both in the Earth and Mars), having been alone glaciated 

 during periods of high excentricity.^ 



Before, however, we can draw any conclusion from the case 

 of Mars, we must carefully scrutinise the facts, and the condi- 

 tions they imply. In the first place, there is evidently this 

 radical difference between the state of Mars now and of the 

 Earth during a glacial period — that Mars has no great ice- 

 sheets spreading over her temperate zone, as the Earth un- 

 doubtedly had. This we know from the fact of the rapid 



1 In an article in Nature of Jan. 1, 1889, the Rev. T. W. Webb states that 

 in 1877 the pole of Mars (? the south pole) was, according to^ Schiaparelli, 

 entirely free of snow. He remarks also on the regular contour of the sup- 

 posed snows of Mars as offering a great contrast to ours, and also the 

 strongly marked dark border which has often been observed. On the whole 

 Mr. Webb seems to be of opinion that there can be no really close resem- 

 blance between the physical condition of the Earth and Mars, and that any 

 arguments founded on such supposed similarity are therefore untrustworthy. 



