MILL, is a machine, which, by means of any adequate force, as steam, water, wind, or animal exertion, acquires such an additional power as enables the machi nery to act with increased effect, and with the requisite regularity. It may be considered an axiom, from which very few, if any, deviations are to be found, that the nearer the labouring part is to the power or origin of motion, the great er will be the force employed; but that force will be subject to such fluctuations, and to such shocks and vibrations, as to render the work inaccurate and unequal. In some operations this is of less import ance than in such as are connected with minute and delicate manufactures. Thus, in brass, copper, and fulling-mills, the large hammers which perform the heavy work derive their motion immediately from the great wheel which is turned by the stream ; they being lifted by cogs, or teeth, set on its axis, and working without any intermediate machinery. In saw-mills, the blades which cut the timbers into planks, he. are removed from the great wheel by the intervention of other wheels, which not only give increased velocity, but relieve the saws from those jerks and strains to which they would be subject, if deprived of those movements which ren der their 'own operations equable and firm. In corn-mills the velocity is again increased by another course added to the system, while the action of the grinding parts is thus effectually discharged from all inequalities of motion; and thus flour, &c. may be ground with certainty to any desired degree of fineness, according as the distances between the stones may be regulated. With regard to the more complex movements required in the ma _ nufacture of silk, cotton, &c., many of them are so remotely connected with the moving power, that they may be made to revolve with the most wonderful exact ness. It is, however, necessary, that the frame work of a mill should be very firm and substantial ; that the pillions, spin dles, and axles, all move freely in their sockets, which should be exactly at right angles with them ; and that no greater pressure should take place on any part than the duty it is to perform may render necessary. These great principles are
inseparable from the proper construction of mills. We consider a knowledge of the powers of mechanism to be absolutely indispensable towards a due understand ing of this subject, and recommend the student to refer to the head of MECH.., 2f ice, for such information as may enable him to form a more correct judgment of the particulars relating to mill-work in general.
With respect to machinery moved by steam, we need say little in this place, since the movements dependent on that kind of power may be found under that article, while the remote or subordinate parts will be seen in the constrpction of such mills as derive their action from wind or water. The selection of the power is not always within our choice ; but must depend on the abundance of fuel, the supply of water, and the due elevation of the spot where the mill is to be erected. Where coals are cheap, the steam engine, being so immensely forci ble, and capable of any desirable bulk, is in most instances preferred. Where fuel is dear, and that a stream of adequate size can be applied, water becomes the mo mentum ; but where neither of the fore going can be found under suitable cir cumstances, a well exposed spot is ordi narily selected for the erection of a wind mill. We shall skew the quantity of wa ter necessary to work a wheel of certain diameters ; observing that eighteen feet has been found from experience to be the most commodious measurement, as well as sufficiently powerful for any overshot. mill ; indeed for breast-mills, that diame ter may be considered as capable of giv ing motion to all the ordinary systems of machinery. It should be observed, that the breadth of the water-wheel ought to correspond with the power necessary on the occasion, supposing that a propor tionate volume of water is at command ; for a wheel of two feet in breadth will be more than doubly as powerful as one of only a foot in breadth ; there being a dog.