46.  A small moving power, in the shape of a jack or a spring with a train of wheels, is often of great convenience to the experimental philosopher, and has been used with advantage in magnetic and electric experiments where the rotation of a disk of metal or other body is necessary, thus allowing to the enquirer the unimpeded use of both his hands.  A vane connected by a train of wheels, and set in motion by a heavy weight, has also, on some occasions, been employed in chemical processes, to keep a solution in a state of agitation.  Another object to which a similar apparatus may be applied, is the polishing of small specimens of minerals for optical experiments.

Chapter 6

Saving time in Natural Operations

47.  The process of tanning will furnish us with a striking illustration of the power of machinery in accelerating certain processes in which natural operations have a principal effect.  The object of this art is to combine a certain principle called tanning with every particle of the skin to be tanned.  This, in the ordinary process, is accomplished by allowing the skins to soak in pits containing a solution of tanning matter:  they remain in the pits six, twelve, or eighteen months; and in some instances (if the hides are very thick), they are exposed to the operation for two years, or even during a longer period.  This length of time is apparently required in order to allow the tanning matter to penetrate into the interior of a thick hide.  The improved process consists in placing the hides with the solution of tan in close vessels, and then exhausting the air.  The effect is to withdraw any air which may be contained in the pores of the hides, and to aid capillary attraction by the pressure of the atmosphere in forcing the tan into the interior of the skins.  The effect of the additional force thus brought into action can be equal only to one atmosphere, but a further improvement has been made:  the vessel containing the hides is, after exhaustion, filled up with a solution of tan; a small additional quantity is then injected with a forcing-pump.  By these means any degree of pressure may be given which the containing vessel is capable of supporting; and it has been found that, by employing such a method, the thickest hides may be tanned in six weeks or two months.

48.  The same process of injection might be applied to impregnate timber with tar, or any other substance capable of preserving it from decay, and if it were not too expensive, the deal floors of houses might thus be impregnated with alumine or other substances, which would render them much less liable to be accidentally set on fire.  In some cases it might be useful to impregnate woods with resins, varnish, or oil; and wood saturated with oil might, in some instances, be usefully employed in machinery for giving a constant, but very minute supply of that fluid to iron or steel, against which it is worked.  Some idea of the