When we expose
the sails of a windmill obliquely to the gale, we check the
velocity of a small portion of the atmosphere, and convert its
own rectilinear motion into one of rotation in the sails; we thus
change the direction of force, but we create no power. The same
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.
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