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 perihelion ), 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 
