5 
16 Rev. FE. Hill—Eccentricity and Glacial Epochs. 
volume. Let us consider the whole year’s cycle of action. Suppose 
the earth’s present orbit to be suddenly changed into the necessary 
shape for maximum eccentricity and winter in aphelion. During the 
winter half of the year there would be far more cold in Northern 
regions than now, and a greater difference between the temperatures 
of Equator and the Pole.! The trade-winds are supposed to depend 
for their intensity on this difference. They would be intensified and 
act as Dr. Croll requires. But during the summer half of the year 
there would be as much more heat in Arctic latitudes than now, and 
a less difference between Equator and Pole. The trade-winds would 
be feebler to a corresponding degree. The Equatorial current would 
not lie so far south as it does. More of it would pass into the Carri- 
bean sea, and the Gulf Stream would carry more heat into the 
Atlantic, thus ameliorating the climates of its coasts. Summer 
effects and winter effects precisely balance, and equilibrium is undis- 
turbed. Nor is the slightest difference made if any part of the cold 
be locked up in the form of ice. This ice in melting absorbs heat 
and lowers or retards the rise of temperature, but in forming it parts 
with exactly the same amount, and correspondingly raises the tem- 
perature of the air or retards its fall. 
We may put the argument into a compacter form. No matter the 
way in which heat may produce air currents, equal quantities of heat 
must produce equal quantities of work, and therefore equal effects 
on currents. Change of eccentricity does not (appreciably) alter the 
total amounts of heat received at any place during a whole year, 
though it may alter the distribution through the year. ‘Thus it 
cannot alter the whole effect on currents, though they may be some- 
times accelerated, sometimes retarded. 
We have thus gone through Dr. Croll’s four causes, or rather 
modes of action, whereby high eccentricity might, as he maintains, 
produce a Glacial Period. The first is non-existent. The second 
may possibly have an effect. The third at best cannot work at all, 
and almost certainly works in the wrong direction. The fourth 
might work if set going, but has no power of starting itself. It 
could, perhaps, be put in motion by aid of the second. But against 
this must be set, not only the probability of a counteraction by 
augmented velocity and the opposite effect of the third, but also the 
real though small increase of total annual heat received during 
increase of eccentricity. This we have hitherto neglected, but it 
exists, and must have its effect, which must be to diminish perma- 
nent snow. Dr. Croll dismisses it without discussion (note, p. 58), 
but, unlike the others, it indisputably acts. 
It should be noticed that though for convenience we have assumed 
that at the present epoch the Arctic snow cap is neither increasing 
nor decreasing, yet this assumption is entirely unnecessary. The 
above investigations show that, whatever be the present state of the 
case, change in eccentricity has not been proved by Dr. Croll to have 
any power of changing that state. 
1 Would this happen? The increased distance of the sun would affect Equator as 
well as Pole. 
