may be observed with regard to the sails of a vessel; the
quantity of motion given by them is precisely the same as that
which is destroyed in the atmosphere. If we avail ourselves of a
descending stream to turn a water-wheel, we are appropriating a
power which nature may appear, at first sight, to be uselessly
and irrecoverably wasting, but which, upon due examination, we
shall find she is ever regaining by other processes. The fluid
which is falling from a higher to a lower level, carries with it
the velocity due to its revolution with the earth at a greater
distance from its centre. It will therefore accelerate, although
to an almost infinitesimal extent, the earth's daily rotation.
The sum of all these increments of velocity, arising from the
descent of all the falling waters on the earth's surface, would
in time become perceptible, did not nature, by the process of
evaporation, convey the waters back to their sources; and thus
again, by removing matter to a greater distance from the centre,
destroy the velocity generated by its previous approach.

17. The force of vapour is another fertile source of moving
power; but even in this case it cannot be maintained that power
is created. Water is converted into elastic vapour by the
combustion of fuel. The chemical changes which thus take place
are constantly increasing the atmosphere by large quantities of
carbonic acid and other gases noxious to animal life. The means
by which nature decomposes these elements, or reconverts them
into a solid form, are not sufficiently known: but if the end
could be accomplished by mechanical force, it is almost certain
that the power necessary to produce it would at least equal that
which was generated by the original combustion. Man, therefore,
does not create power; but, availing himself of his knowledge of