Physical Theory of Secular Changes of Climate. 189 



other we could remove the snow and ice from the arctic regions, 

 they would then enjoy a temperate, if not a hot summer. In 

 Greenland, as we have already seen, even in the very middle of 

 summer snow falls, more or less, nine days out of ten*; but re- 

 move the snow from the northern hemisphere, and a snow- 

 shower in Greenland during summer would be as great a rarity 

 as it would be on the plains of India. 



Other things being equal, the quantity of solar heat received 

 at Greenland during summer is considerably greater than in 

 England. Consequently, were it not for snow and ice, it would 

 enjoy as warm a climate during summer as that of England. 

 Conversely, let the polar snow and ice extend to the latitude of 

 England, and the summers of that country would be as cold as 

 those of Greenland. Our summers would then be as cold as our 

 winters are at present, and snow in the very middle of summer 

 would perhaps be as common as rain. 



How the foregoing causes deflect Ocean-currents. — We shall now 

 see the consequences to which all this leads. A high condition 

 of eccentricity tends, we have seen, to produce an accumulation 

 of snow and ice on the hemisphere whose winters occur in aphe- 

 lion. The accumulation of snow in turn tends to lower the 

 summer temperature, cut off the sun's rays, and retard the 

 melting of the snow. In short, it tends to produce on that he- 

 misphere a state of glaciation. Exactly opposite effects take place 

 on the other hemisphere, which has its winter in perihelion. 

 There the shortness of the winters and the highness of the tem- 

 perature, owing to the sun's nearness, tend to prevent the accu- 

 mulation of snow. The general result is that the one hemi- 

 sphere is cooled and the other heated. This state of things now 

 brings into play the agencies which lead to the deflection of the 

 Gulf-stream and other great ocean-currents. 



Owing to the great difference between the temperature of the 

 equator and the poles, there is a constant flow of air from the 

 poles to the equator. It is to this that the trade-winds owe 

 their existence. Now as the strength of these winds will, as a 

 general rule, depend upon the difference of temperature that 

 may exist between the equator and higher latitudes, it follows 

 that the trades on the cold hemisphere will be stronger than 

 those on the warm. When the polar and temperate regions of 

 the one hemisphere are covered to a large extent with snow and 

 ice, the air, as we have just seen, is kept almost at the freezing- 

 point during both summer and winter. The trades on that he- 

 misphere will of necessity be exceedingly powerful; while on 

 the other hemisphere, where there is comparatively little snow 

 and ice and the air is warm, the trades will, as a consequence, be 

 * Arctic Regions, vol. i. p. 425. 



