Correspondence — W. H. 8. Monck. 333 



heavy fall that the temperature should not be much below freezing- 

 point. 



I now distinguish three cases. 



1st. Where the mean temperature is above freezing-point. Here, 

 if we could distribute equally throughout the year, no snow would 

 fall. Unequal distribution might, however, produce a considerable 

 snow-fall, though not a permanent snow-cap. In mountainous 

 districts extensive glaciers might be produced in this way. 



2nd. Where the mean temperature is below, but not much below, 

 freezing-point. Here an equal distribution of heat throughout the 

 year is most favourable to the formation of a snow-cap. Snow 

 would fall at all seasons of the year, and the melting-point being 

 rarely, if ever, attained, the snow-cap would continue to accumulate. 



3rd. Where the temperature is much below freezing-point. Here 

 an unequal distribution of heat is most favourable to glaciation, 

 because we must bring the temperature nearly up to freezing-point 

 at one season of the year in order to obtain the heavy falls of snow 

 which are required to form a snow-cap or ice-cap of considerable 

 thickness. 



If these principles are correct, they lead to the following results. 

 A high eccentricity of the earth's orbit when the earth is in aphelion 

 at mid-winter is favourable to glaciation in two regions of the 

 Northern Hemisphere, one immediately round the pole and the 

 other much further south (whei'e, however, the result would be 

 rather extensive detached glaciers than general glaciation). But 

 between these two regions there is an intervening one in which the 

 conditions for glaciation would be unfavourable, the snow-fall being 

 less than if the distribution of heat was equable, while a good deal 

 of this lessened snow-fall would be melted by the increased quantity 

 of heat received during the summer. If the earth was in perihelion 

 at mid -winter, this state of things would be reversed. The polar 

 snow-cap would be diminished, but there would be more glaciation 

 in the southern portion of the Arctic region and the northern portion 

 of the Temperate Zone. As far south as Switzerland, however, the 

 glaciation might perhaps again diminish. 



In confirmation of these views, I may add that I do not see how 

 the snow-fall could be increased over the entire region from, say, the 

 fiftieth degree of North Latitude to the Pole at the same time. For 

 high eccentricity would not, I apprehend, increase the difference of 

 temperatures between the Equatorial and Polar Eegions. It would 

 produce a summer and winter at the Equator — the former when the 

 earth was in perihelion and the latter when the earth was in aphelion ; 

 but when the Equatorial and Polar summers and winters synchronized, 

 the contrast of temperatures would not be greater than at present. 

 During the long cold northern winter, on which Dr. Croll lays so 

 much stress, there would also be winter at the Equator, and if we 

 regard the Equatorial region as the generator of vapour or steam and 

 the Polar region as the condenser, the apparatus as a whole would not 

 be more powerful but less powerful than before. There would be 

 no increase in the quantity of aqueous vapour available for the 



