t biology, in the long run, rests upon physics, and the first condition
for arriving at a sound theory of distribution in the deep sea, is the
precise ascertainment of the conditions of life; or, in other words, a
full knowledge of all those phenomena which are embraced under the head
of the Physical Geography of the Ocean.

Excellent work has already been done in this direction, chiefly under the
superintendence of Dr. Carpenter, by the _Lightning_ and the
_Porcupine_,[10] and some data of fundamental importance to the physical
geography of the sea have been fixed beyond a doubt.

[Footnote 10: _Proceedings of the Royal Society_, 1870 and 1872]

Thus, though it is true that sea-water steadily contracts as it cools
down to its freezing point, instead of expanding before it reaches its
freezing point as fresh water does, the truth has been steadily ignored
by even the highest authorities in physical geography, and the erroneous
conclusions deduced from their erroneous premises have been widely
accepted as if they were ascertained facts. Of course, if sea-water, like
fresh water, were heaviest at a temperature of 39 deg. F. and got lighter as
it approached 32 deg. F., the water of the bottom of the deep sea could not
be colder than 39 deg.. But one of the first results of the careful
ascertainment of the temperature at different depths, by means of
thermometers specially contrived for the avoidance of the errors produced
by pressure, was the proof that, below 1000 fathoms in the Atlantic, down
to the greatest depths yet sounded, the water has a temperature always
lower than 38 deg. Fahr., whatever be the temperature of the water at the
surface. And that this low temperature of the deepest water is probably
the universal rule for the depths of the open ocean is shown, among
others, by Captain Chimmo's recent observations in the Indian ocean,
between Ceylon and Sumatra, where, the surface water ranging from 85 deg.-81 deg.
Fahr., the temperature at the bottom, at a depth of 2270 to 2656 fathoms,
was only from 34 deg. to 32 deg. Fahr.

As the mean temperature of the superficial layer of the crust of the
earth may be taken at about 50 deg. Fahr., it follows that the bottom layer
of the deep sea in temperate and hot latitudes, is, on the average, much
colder than either of the bodies with which it is in contact; for the
temperature of the earth is constant, while that of the air rarely falls
so low as that of the bottom wate