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 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-cap was observed during 
its summer, and was found to vary as follows : — 
May 
4th. 
Diameter of spot 
31° 24 
June 
4 th. 
11 
yy 
28° 0' 
yy 
17 th. 
11 
22° 54' 
July 
4th. 
11 
yy 
18° 24' 
>1 
12 th. 
11 
4 ii 
15° 20' 
iy 
20th. 
yy 
18 3 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 aphelion (as the southern now 
has both in the Earth and Mars), having been alone glaciated 
during periods of high excentricity. 1 * 
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, 1880, 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. 
