THE GLACIAL EPOCH. 



27 



the other hand, can it be supposed that any amount of sunshine will melt the snow in a 

 dry air without wind. Any one who has spent a spring in the Arctic Regions, or has 

 visited the southern regions of perpetual snow in summer, must have noticed that, although 

 the sun's rays are hot enough to blister the skin, they seem to glance harmless from the 

 white snow. Dry air is incapable of absorbing heat. Even in windy weather, on some of 

 the highest passes of the Himalayas, the thermometer may stand at freezing-point in the 

 shade, although the sun is so hot that the blackened bulb of a thermometer may register 

 more than two hundred degrees. 



To produce a Glacial Epoch it is necessary either to increase the moisture of the autumn 

 gales, or to decrease that of the spring gales, or, better still, to do both. If the autumn gales 

 are dry, they may blow as hard as they like, but they will deposit little snow ; and if the 

 spring gales are dry they will also be cold, and will melt it very slowly. 



The earth does not revolve round the sun in a circle but in an ellipse, in one of the 

 foci of which the sun is placed. This eccentricity of the earth's orbit varies in amount, 

 according to the relative position of the rest of the planets. When the eccentricity of the 

 earth's orbit is great, the difference in the amount of heat received by the earth from the 

 sun, when the former is in perihelion, and that received when it is in aphelion, is 

 probably sufficient to make an important difference in the amount of moisture held in 

 suspension by the earth's atmosphere at the two periods. When the eccentricity of 

 the earth's orbit is great during the four months or so of perihelion, the hot though 

 short summer produces abundance of vapour, which in autumn is carried by the 

 equinoctial gales towards the Pole entering winter in aphelion, and produces great 

 quantities of snow, whilst at the Pole emerging from winter in perihelion the extra heat 

 and moisture of the air melts the snow rapidly. Six months later the opposite state of 

 things produces the same result. During the half of the earth's progress round the 

 sun, in which it is furthest from that body, the atmosphere is cooler and less charged 

 with moisture. The snow at the Pole emerging from winter in aphelion is much longer 

 in melting in consequence of the dryness and coolness of the south wind ; whilst at 

 the Pole entering winter in perihelion the less amount of moisture in the air causes a 

 smaller deposit of snow. In both cases the same result must be produced, a glacial 

 epoch at the Pole which has its winter in aphelion, and a tropical epoch at the Pole 

 which has its winter in perihelion. If this be the cause of the Glacial Epoch, it follows 

 that the intensity of these periods must increase as the eccentricity of the earth's orbit 

 increases, and they must disappear altogether when the orbit of the earth is circular. The 

 precession of the equinox, which takes 21,000 years to complete, reverses the condition of 

 either Pole ; so that a Glacial Epoch at each Pole lasts for 10,500 years, followed by a tropical 

 epoch of the same duration, and this must be repeated as long as the unusual eccentricity 

 of the earth's orbit continues. 



Wallace, in his ' Island Life,' argues at considerable length against the theory of 

 tropical interglacial periods extending to the Pole during the Glacial Epoch. He appears 



Changes re- 

 quisite to 

 produce a 

 Glacial 

 Epoch. 



Eccentricity 

 of the 

 earth's 

 orbit. 



Effect of the 

 earth in 

 aphelion and 

 in perihe- 

 lion. 



Dependent 

 on the 

 amount of 

 the eccen- 

 tricity. 



