hat for every cubic foot seen above, there must at least
be eight cubic feet below water.[181] If ice islands from the north
polar regions floated as far, they might reach Cape St. Vincent, and
there, being drawn by the current that always sets in from the Atlantic
through the Straits of Gibraltar, be drifted into the Mediterranean, so
that the serene sky of that delightful region might soon be deformed by
clouds and mists.

[Illustration: Fig. 2.

Iceberg seen off the Cape of Good Hope, April, 1829. Lat. 89 degrees 18
minutes S. Long. 48 degrees 46 minutes E.]

Before the amount of difference between the temperature of the two
hemispheres was ascertained, it was referred by many astronomers to the
precession of the equinoxes, or the acceleration of the earth's motion
in its perihelium; in consequence of which the spring and summer of the
southern hemisphere are now shorter, by nearly eight days, than those
seasons north of the equator. But Sir J. Herschel reminds us that the
excess of eight days in the duration of the sun's presence in the
northern hemisphere is not productive of an excess of annual light and
heat; since, according to the laws of elliptic motion, it is
demonstrable that whatever be the ellipticity of the earth's orbit, the
two hemispheres must receive _equal absolute quantities_ of light and
heat per annum, the proximity of the sun in perigee exactly compensating
the effect of its swifter motion.[182] Humboldt, however, observes, that
there must be a greater loss of heat by radiation in the southern
hemisphere during a winter longer by eight days than that on the other
side of the equator.[183]

Perhaps no very sensible effect may be produced by this source of
disturbance; yet the geologist should bear in mind that to a certain
extent it operates alternately on each of the two hemispheres for a
period of upwards of 10,000 years, dividing unequally the times during
which the annual supply of solar light and heat is received. This cause
may sometimes tend to counterbalance inequalities of temperature
resulting from other far more influential circumstances; but, on the
other hand, it must sometimes tend to increase the extreme of deviation
arising from particular combinations of causes.

But whatever may be at present the inferiority of heat in the temperate
and frigid zones south of the line, it is quite evident that the cold
would be far more intense if there happened, instead of open sea, to be
t