12 Rev. E. Hill—Eccentricity and Glacial Epochs. 
small difference made by change of eccentricity in the whole amount 
of heat annually received from the sun, and consider that total un- 
alterable. If the eccentricity increase, the earth draws nearer the 
sun in the one half the year, is further from him in the other half. 
If midsummer coincide with perihelion, the increase will cause more 
heat to be received by the hemisphere in the summer half of the 
year, and less heat in the winter half. But since the total remains 
the same, the excess in one half is equal to the defect in the other. 
The amount of heat received in the year is unaltered; it is only 
differently distributed. Dr. Croll’s theorem, therefore, may be stated 
thus,—A Glacial period may be produced by a different distribution 
of the fixed amount of heat received annually, and the most favour- 
able distribution is that by which as much as possible is concentrated 
into one-half the year. 
We will proceed to examine this theorem, and the arguments 
which are adduced as a demonstration. We must bear in mind that 
the Glacial period to be accounted for is not simply a cold winter, 
or a succession of such, but a period during which ice and snow 
accumulated toa vast mass, permanently covering our country during 
summer as well as winter: a time when a glacier 1000 feet thick 
lay in the pass of Llanberis, and the ice-sheet from Scandinavia 
(according to Dr. Croll) ground down the Orkneys and the Hebrides. 
Every one admits that these vast amounts could only arise by suc- 
cessive increase during long periods of years. Dr. Croll’s explana- 
tion is that, during periods of high eccentricity, such quantities of 
snow and ice will be formed in the long severe winter, as the short 
summer will be unable to melt. I always feel bewildered at his 
beginning with the winter. Has not summer as much right to be 
considered ? Why not say that in the intense heat of summer more 
polar ice will be melted than now? We might exactly reverse his 
argument. Yes, says Dr. Croll, but the winter is longer. But I 
must rejoin, what have short and long to do with the question ? 
The quantities of heat are unaltered, he admits. Quantities of heat, 
not lengths of time, are what we ought to look at. To melt a mass 
of ice, a definite quantity of heat must be supplied; to form the 
same mass, an equal quantity must be subtracted. The amount of 
heat received annually will not be less than at present; no reason 
is given why the amount given off should increase. If snow is to 
begin to accumulate, more must be formed than is melted; then 
either more heat must be lost annually, or else less received than is 
now the case. But how will increased eccentricity be able to bring 
about either of these effects ? 
The matter will be made more clear by dividing the year into two 
equal halves, as I described above. We have no reason to suppose 
that at present in the Northern Hemisphere more snow or ice is any- 
where formed in winter than is melted in the summer. With 
greater eccentricity less heat than now would be received in winter, 
but exactly as much more in summer. More snow would therefore 
be formed in the one half of the year, but exactly as much more be 
melted in the other half. The colder winter and the warmer 
