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. 



Froperties 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. CrolFs tlieory 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. 



