e of the small superiority in saline contents of the
Mediterranean water in some condition other than solar evaporation.

Again, if the Gibraltar indraught is the effect of evaporation, why does
it go on in winter as well as in summer?

All these are questions more easily asked than answered; but they must be
answered before we can accept the Gibraltar stream as an example of a
current produced by indraught with any comfort.

The Mediterranean is not included in the _Challenger's_ route, but she
will visit one of the most promising and little explored of
hydrographical regions--the North Pacific, between Polynesia and the
Asiatic and American shores; and doubtless the store of observations upon
the currents of this region, which she will accumulate, when compared
with what we know of the North Atlantic, will throw a powerful light upon
the present obscurity of the Gulf-stream problem.

III

ON SOME OF THE RESULTS OF THE EXPEDITION OF H.M.S. _CHALLLENGER_

[1875]

In May, 1873, I drew attention[1] to the important problems connected
with the physics and natural history of the sea, to the solution of which
there was every reason to hope the cruise of H.M.S. _Challenger_ would
furnish important contributions. The expectation then expressed has not
been disappointed. Reports to the Admiralty, papers communicated to the
Royal Society, and large collections which have already been sent home,
have shown that the _Challenger's_ staff have made admirable use of their
great opportunities; and that, on the return of the expedition in 1874,
their performance will be fully up to the level of their promise. Indeed,
I am disposed to go so far as to say, that if nothing more came of the
_Challengers_ expedition than has hitherto been yielded by her
exploration of the nature of the sea bottom at great depths, a full
scientific equivalent of the trouble and expense of her equipment would
have been obtained.

[Footnote 1: See the preceding Essay.]

In order to justify this assertion, and yet, at the same time, not to
claim more for Professor Wyville Thomson and his colleagues than is their
due, I must give a brief history of the observations which have preceded
their exploration of this recondite field of research, and endeavour to
make clear what was the state of knowledge in December, 1872, and what
new facts have been added by the scientific staff of the _Challenger_. So
far as I have been able to discover, the first su