chap, viii.] THE CAUSES OF GLACIAL EPOCHS. 
127 
be long continued ; but at the same time we should have almost 
tropical heat in summer, although that season would be some- 
what shorter. How then, it may be asked, could such a climate 
have the effect supposed ? Would not the snow that fell in 
winter be all melted by the excessively hot summer ? In order 
to answer this question we must take account of certain 
properties of water and air, snow and ice, to which due weight 
has not been given by writers on this subject. 
Properties of Air and Water , Snow and Ice , in relation to 
Climate . — The great aerial ocean which surrounds us has the 
wonderful property of allowing the heat-rays from the sun to 
pass through it without its being warmed by them ; but when 
the earth is heated the air gets warmed by contact with it, and 
also to a considerable extent by the heat radiated from the 
warm earth, because, although pure dry air allows such dark 
heat-rays to pass freely, yet the aqueous vapour and carbonic 
acid in the air intercept and absorb them. But the air thus 
warmed by the earth is in continual motion owing to 
changes of density. It rises up and flows off, while cooler air 
supplies its place ; and thus heat can never accumulate in the 
atmosphere beyond a very moderate degree, the excessive sun- 
heat of the tropics being much of it carried away to the upper 
atmosphere and radiated into space. Water also is very mobile; 
and although it receives and stores up a great deal of heat, it 
is for ever dispersing it over the earth. The rain which brings 
down a certain portion of heat from the atmosphere, and which 
often absorbs heat from the earth on which it falls, flows away 
in streams to the ocean ; while the ocean itself, constantly im- 
pelled by the winds, forms great currents, which carry off the 
adoption of Dr. Croll’s theory of the Astronomical and Physical causes of 
the Glacial Epoch. 
The reason of the increase of summer heat being 60° while the decrease 
of winter cold is only 35°, is because our summer is now below and our 
winter above the average. A large part of the 60° increase of temperature 
would no doubt be used up in evaporating water, so that there would be a 
much less increase of sensible heat ; while only a portion of the 35° 
lowering of temperature in winter would be actually produced owing to 
equalising effect of winds and currents, and the storing up of heat by 
the earth and ocean. 
