FROM THE CHALK OF HERTFORDSHIRE. 
133 
Even snow protects the ground from frost, being, owing to the 
air confined within its interstices, a had conductor of heat; and 
without the heat of the sun, the vapour required for the formation 
of snow could not rise. It has, nevertheless, been assumed that 
the Glacial period must have been one of general cold, and might 
therefore be due, in part at least, to the earth receiving from the 
sun a less amount of heat than it now does. But the fact is that 
the contrary is more likely to have been the case, for the abundance 
of snow depends upon the humidity of the air, and therefore the 
immense amount of snow that must have fallen to form an ice-cap 
over the greater part of Northern Europe necessitates a great 
heating power of the sun to raise the amount of vapour required 
to form that snow. 
The source of all water-supply is rain, and in this country the 
chief characteristic of rainfall is its great variability. This masks 
any secular variation there may be in our time, and baffles all 
attempts at prediction from one year to another, and almost from 
one day to the next. We have every reason to believe, however, 
that the rainfall on our globe is decreasing. We infer it from 
what we know must have been the early history of the earth, and 
we have both historical and geological evidence of it. The source 
of all rainfall is evaporation, which is dependent upon the heating 
power of the. sun. But it is not to the secular cooling of the sun, 
of which we have no certain evidence, that I am alluding, but to 
that of the earth, of which we have precise data; and also to 
alterations in the configuration of the earth’s surface. 
Presuming that the total amount of water in the atmosphere, on 
the surface of the ground, as in rivers, lakes, and seas, and within 
its crust, is a constant quantity, when the earth was very highly 
heated the whole of it would be in the atmosphere; as the surface 
slowly cooled, the water which fell upon it would no longer be 
driven off as steam, but would accumulate in seas and run off the 
higher land as rivers into these seas, but none would yet be able 
to percolate into the rocks, owing to their still being in a heated 
condition. There would then be a more rapid circulation of water 
between the earth and its atmosphere than there is now ; in other 
words there would be more evaporation and more rain. As the 
earth continued to cool, water would gradually percolate into its 
porous strata, but would be driven off whenever it reached the 
plane of temperature of its boiling-point, this plane getting gradu¬ 
ally lower and lower, until now it is at a distance of about 10,000 
feet from the surface, where the boiling-point of water will be 
considerably higher than 212°. Our constant quantity of water, 
all at one time in the atmosphere, thus first became divided between 
the atmosphere and the surface of the earth, and then between the 
atmosphere and the surface and interior of the earth, and with 
a gradually lessening quantity above the surface, there must be a 
gradually decreasing evaporation and rainfall. 
This cause of a decreasing rainfall is cosmical, and it will, with¬ 
out a reasonable doubt, eventually reduce our earth to the same 
