
ral 
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Scr, HI.] CLIMATE IN ITS GEOLOGICAL RELATIONS. 25 
summers; but with the eccentricity at its superior limit and the 
winter solstice in aphelion, the length of the winters would exceed 
that of the summers by no fewer than thirty-six days. The lowering 
of the temperature and the lengthening of the winter would both 
tend to the same effect, viz., to increase the amount of snow accumu- 
lated during the winter; for, other things being equal, the larger 
the snow-accumulating period the greater the accumulation. It may 
be remarked, however, that the absolute quantity of heat received 
during winter is not affected by the decrease in the sun’s heat,’ for 
the additional length of the season compensates for this decrease. 
As regards the absolute amount of heat received, increase of the sun’s 
distance and lengthening of the winter are compensatory, but not so 
in regard to the amount of snow accumulated. ‘The consequence of 
this state of things would be that, at the commencement of the short 
summer, the ground would be covered with the winter’s accumula- 
tion of snow. Again, the presence of so much snow would lower the 
summer temperature, and prevent to a great extent the melting of 
the snow. | 
«There are three separate ways whereby accumulated masses of 
snow and ice tend to lower the summer temperature, viz. :— 
“ Hirst, By means of direct radiation. No matter what the 
intensity of the sun’s rays may be, the temperature of snow and ice 
can never rise above 32°. Hence the presence of snow and ice tends 
by direct radiation to lower the temperature of all surrounding bodies 
to 32°. In Greenland, a country covered with snow and ice, the 
pitch has been seen to melt on the side of a ship exposed to the © 
direct rays of the sun, while at the same time the surrounding air was 
far below the freezing point; a thermometer exposed to the direct 
radiation of the sun has been observed to stand above 100°, while 
the air surrounding the instrument was actually 12° below the 
freezing-point. A similar experience has been recorded by travellers 
on the snow-fields of the Alps. ‘These results, surprising as they no 
doubt appear, are what we ought to expect under the circumstances. 
Perfectly dry air seems to be nearly incapable of absorbing radiant 
heat. The entire radiation passes through it almost without any 
sensible absorption. _ Consequently the pitch on the side of the ship 
may be melted, or the bulb of the thermometer raised to a high 
temperature by the direct rays of the sun, while the surrounding 
air remains intensely cold. ‘The air is cooled by contact with 
the snow-covered ground, but is not heated by the radiation from 
_the sun. 
“ When the air is charged with aqueous vapour, a similar cooling 
effect also takes place, but in a slightly different way. Air charged 
with aqueous vapour is a good absorber of radiant heat, but it can 
only absorb those rays which agree with it in period. It so happens 
1 When the eccentricity is at its superior limit, the absolute quantity of heat received 
by the earth during the year is, however, about one three-hundredth part greater than 
at present. But this does not affect the question at issue, 
