430 Climate of the Glacial Period. [Oétober, 
evolves just as much heat as is required to melt it again, 
and the heat given off in winter by the freezing water is 
equal to that absorbed when it melts again, so that the mean 
temperature is not affected. 
Again, it is said that clouds would accumulate around the 
pole with its winter in aphelion. Why they should do so 
does not appear very clearly, but clouds would*receive the 
rays of the sun on their upper surface, and in some way or 
other the heat would be utilised in ameliorating the climate ; 
nor should it be forgotten that clouds prevent radiation 
during the night as well as intercept the sun’s rays during 
the day. 
There is, however, a cause not touched upon by Mr. 
Croll that does act in preventing the excess of heat of 
summer counterbalancing its diminution in winter where 
snow covers the ground. It is not because the heat is 
used up in melting the snow, but because much of it is 
not so used up but is reflected back into space from the 
white surface. If it were not for this, snow would nowhere 
be perennial, but everywhere the heat of summer would 
dissolve the snows of winter; and if, without taking into 
account any lengthening of the winter by reason of the 
ellipticity of the orbit, the whole of the winter solstice were 
an ice-accumulating period it would now gather year by 
year until it overwhelmed the temperate zones, because the six 
months’ snow would reflect so much of the other six months’ 
heat that it would not be melted but would gradually ac- 
cumulate. It does not do so, because only at the very 
poles are there six months winter and six months summer, 
and the ice-accumulating period gradually decreases when 
we leave the poles, and reaches zero long before we arrive 
at the tropics. These conditions were probably the same 
at the time of greatest ellipticity, and at the most onlya 
very small amount of heat could be lost by reflection from 
snow-covered lands more than now: and as at that time— 
according to the law that the amount of heat received is in 
inverse proportion to the length of the shortest diameter of 
the orbit—there would be a slight increase in the absolute 
amount of heat received from the sun, it is probable that 
one would counterbalance the other; and I cannot but come 
to the conclusion that Arago was right when he affirmed 
that even if the ellipticity of the orbit was much greater 
than astronomers have shown to be possible, ‘‘still this 
would not alter in any appreciable manner the mean 
thermometrical state of the globe.” 
Mr. Croll has sought to strengthen his theory by 
