16.  Of those machines by which we produce power, it may be observed, that although they are to us immense acquisitions, yet in regard to two of the sources of this power—­the force of wind and of water—­we merely make use of bodies in a state of motion by nature; we change the directions of their movement in order to render them subservient to our purposes, but we neither add to nor diminish the quantity of motion in existence.  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.

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 nature’s mysteries, he applies his talents to diverting a small and limited portion of her energies to his own wants:  and, whether he employs the regulated action of steam, or the more rapid and tremendous effects of gunpowder, he is only producing on a small scale compositions and decompositions which nature is incessantly at work in reversing, for the restoration of that equilibrium which we cannot doubt is constantly maintained throughout even the remotest limits of our system.  The operations of man participate in the character of their author; they are diminutive, but energetic during the short period of their existence:  whilst those of nature, acting over vast spaces, and unlimited by time, are ever pursuing their silent and resistless career.