152 



ISLAND LIFE. 



[part I. 



from producing any effect, by reflection from the surface of the 

 snow and by the intervention of clouds and fog which would 

 carry much of the heat they received to other regions. It 

 follows that 10,000 years hence, when our winter occurs in 

 aphelion (instead of, as now, in pei^ihelion), there will be produced 

 -a colder climate, independently of any change of land and sea, 

 of heights of mountains, or the force of currents. 



But if this is true, then the reverse change, bringing the sun 

 back into exactly the same position with regard to us as it is in 

 now (all geographical and physical conditions remaining un- 

 changed), would certainly bring back again our present milder 

 climate. The change either way would not probably be very 

 great, but it might be sufficient to bring the snow line down to 

 3,000 feet in Scotland, so that all the higher mountains had 

 their tops covered with perpetual snow. This perpetual snow, 

 down to a fixed line, would be kept up by the necessary supply 

 of snow falling during autumn, winter, and spring, and this 

 would, as we have seen, depend mainly on the increased length 

 and greatly increased cold of the winter. As both the duration 

 and the cold of winter decreased the amount of snow would 

 certainly decrease, and of this lesser quantity of snow a larger 

 proportion would be melted by the longer, though somewhat 

 cooler summer. This would follow because the total amount of 

 sun-heat received during the summer would be the same as 

 before, while it would act on a less quantity of snow ; there 

 would thus be a smaller surface to reflect the heat, and a smaller 

 condensing area to produce fogs, while the diminished intensity 

 of the sun would produce a less dense canopy of clouds, which 

 have been shown to be of prime importance in checking the 

 melting of snow by the sun. We- have considered this case, for 

 simplicity of reasoning, on the supposition that all geographical 

 and physical causes remained unchanged. But if an alteration 

 of the climate of the whole north temperate and Arctic zones 

 occurred, as here indicated, this would certainly affect both the 

 winds and currents, in the manner already explained {see p. 137), 

 so as to react upon climate and increase the differences produced 

 by phases of precession. How far that effect would be again 

 increased by corresponding but opposite changes in the southern 



