348 Glacial Period in the [July, 
judging from what we know of the slope of the upper 
surfaces of the present ice-sheets of Greenland and Spitz- 
bergen, and from the marks left by the old Alpine glaciers, 
we shall not, I think, be justified in allowing the still larger 
circumpolar ice-sheets a greater surface slope than i in 200; 
and supposing that the ice was 2000 feet higher than the 
present level of the sea, when it reached New Zealand, we 
should have to go back in the direction from which it flowed 
more than 200 miles before we reached a height of 8 coo feet 
above the present sea-level, which may have been the zone 
of greatest precipitation. 
Prof. Tyndall, in his well-known and often-quoted ob- 
servations on the Glacial period, has so forcibly and clearly 
defined some of the necessary conditions to allow of a great 
accumulation of ice, that I may be allowed again to refer to 
them. He shows that the ancient glaciers required for their 
formation that water should be evaporated by heat, and that 
by lessening the force of the sun’s rays we should not increase 
the glaciers, but cut them off at their source ; and he illus- 
trates this by referring to a distilling apparatus, and re- 
minding us that if we wished to increase the quantity 
distilled we should certainly not attain our objeCt by 
reducing the fire under our boiler. “ It is quite manifest,” 
he says, “ that the thing most needed to produce the gla- 
ciers is an improved condenser ; we cannot afford to lose an 
iota of solar adtion ; we need, if anything, more vapour, 
but we need a condenser so powerful that this vapour, 
instead of falling in liquid showers to the earth, shall be so 
far reduced in temperature as to descend in snow.”* If the 
theory I am advocating is corredt the great condenser that 
exists within the Antardtic circle moved northward, and as 
it so moved the vapour to be condensed increased in quan- 
tity, for the reasons I have already given. It was prevented 
from falling in liquid showers, though it did not reach the 
earth as snow, but as ice flowing from the icy ridge that 
was fed and capped with snow. 
Air, as it rises, expands, and much of its heat becomes 
latent, so that it is cooled according to a definite and well- 
ascertained law. To speak popularly, there is an inex- 
haustible store of cold up above, available for glaciation, if 
it could only be brought down.! The vapour in the air is 
chilled when it rises to a great height through the abstraction 
of its heat by the expanding air. The vapour is thus con- 
densed, and at high altitudes frozen, so that it falls as snow. 
* Heat as a Mode of Motion, p. 188. 
| I am indebted to Prof. Joseph Henry, of Washington, for this suggestion. 
