The Pacific Ocean and the Indian Ocean may be considered as one sheet
of water covering an area quite equal in extent to one half of that
embraced by the whole surface of the earth; and the total annual fall
of rain on the earth’s surface is 186,240 cubic imperial miles. Not
less than three-fourths of the vapour which makes this rain comes from
this waste of waters; but, supposing that only half of this quantity,
that is 93,120 cubic miles of rain, falls upon this sea, and that that
much at least is taken up from it again as vapour, this would give
255 cubic miles as the quantity of water which is daily lifted up and
poured back again into this expanse. It is taken up at one place,
and rained down at another; and in this process, therefore, we have
agencies for multitudes of partial and conflicting currents, all, in
their set strength, apparently as uncertain as the winds.
The better to appreciate the operation of such agencies in producing
currents in the sea, imagine a district of 255 square miles to be set
apart in the midst of the Pacific Ocean as the scene of operations
for one day; then conceive a machine capable of pumping up in the
twenty-four hours all the water to the depth of one mile in this
district. The machine must not only pump up and bear off this immense
quantity of water, but it must discharge it again into the sea on the
same day, but at some other place.
All the great rivers of America, Europe, and Asia are lifted up by the
atmosphere, and flow in invisible streams back through the air to their
sources among the hills; and through channels so regular, certain, and
well defined, that the quantity thus conveyed one year with the other
is nearly the same: for that is the quantity which we see running down
to the ocean through these rivers; and the quantity discharged annually
by each river is, as far as we can judge, nearly a constant.--_Maury._