Rev. LE. Hilli—Eccentricity and Glacial Epochs. 15 
Croll’s picture. But there is a companion to it, which he seems not 
to have looked at. Suppose winter coming on. The sun is sending 
little heat. The earth is steadily radiating heat away. Its tempera- 
ture falls. The vapour in the air is condensed into fog, and the fog 
checks radiation from the earth, causing it to cool more slowly. 
Every gardener in fear for his plants, every skater longing for a frost, 
knows that severe cold and a cloudy sky are almost incompatible. 
Fogs retain at least as much heat as they exclude. 
If then clouds and mists and fogs are as capable of checking heat 
radiated from the ground as of intercepting heat received from the 
sun, and if the amount received from the sun does not exceed that 
radiated from the ground, it is impossible that clouds or fogs can 
tend to lower the mean annual temperature. It may, perhaps, be 
thought that a stratum of clouds high above the ground will receive, 
absorb, and retain the heat coming both from earth and sun. True, 
but if they receive heat and are warmed, they can and will radiate 
heat off, both outwards and inwards. Only part of the earth-radia- 
tion intercepted is restored to the earth, but part of the sun-heat 
intercepted is passed on to the ground. However, when heat is in- 
tercepted by mist, its main effect is to convert the mist into trans- 
parent vapour, giving freer passage to radiations. In the formation 
of the cloud, heat is given off from the vapour ; in its reconversion 
into vapour the same amount must be absorbed. Clouds and fogs 
cannot do the work which Dr. Croll requires of them. If high 
eccentricity is to produce a Glacial epoch, they will lend it no aid. 
One action, however, they have; an important one; to which Dr. 
Croll appeals elsewhere (cf. pp. 36, 38, 90 [4]), though here he 
seems to neglect it. The heat received from the sun, and that radiated 
from the earth, though equal in quantity, are not identical in quality. 
The former has greater power of penetration. More of it will pass 
through an interposing medium. The intense sun rays will pene- 
trate fogs which would almost wholly stop the feeble earth-heat. 
Fogs, therefore, do not diminish income and expenditure of heat 
alike. They diminish expenditure more than income, and on the 
whole average of the year actually must tend to increase the tem- 
perature. 
The Fourth, the only remaining cause, is the one on which Dr. 
Croll appears now mainly to rely (cf. for instance, Grou. Mac. 1878, 
p. 8997; 1879, p. 480). Perhaps the most fascinating part of Dr. 
Croll’s whole book is that in which he argues that Polar accumula- 
tions of ice will intensify the North-east Trade-winds, thereby drive 
the Equatorial currents further to the south, diminish the part of 
them which as the Gulf Stream passes into the Atlantic, and so per- 
petuate and intensify the previously increased Polar cold. Nothing 
more original and remarkable occurs in this most remarkable and 
original book. But will the action take place ? 
Let us neglect the disputes concerning the influence of the Gulf 
Stream on climates, and the dependence of currents on winds. Let 
us neglect also the (important) possibility that the greater speed 
which the current would possess might compensate for its decreased 
