MILLS are machines for triturating all kinds of substances capable of being reduced or pulverized by their action. Those employed for converting grain into flour, by rubbing it between two bard surfaces, are generally of stone. The earliest species of mills were of a very rude and simple consisting of two fiat stones, one placed on the other, and the uppermost turned by hand, resembling the figures shown in the following engraving, which represents a band-mill in nearly universal use at the present day amongst the eastern countries. The two stones are put together, as in the figure, and the upper one is then turned by band round the central pivot. Mills Of this description were in common use amongst the Egyptians, Hebrews, Romans, and other nations of antiquity, and continued in use in the Highlands of Scotland until a very recent date ; the principle is indeed the same as that of our most modern and improved mills, but it is only adapted for grinding small quantities of grain at a time.
Under the head HAND-MILLS we have described two mills, which were designed to illustrate a mode of applying manual power to such machines, that has been deemed by various eminent writers on mechanics as the most efficient; namely, that of rowing. But however energetic that action may be, it does not appear to have stood the test of experience ; probably on account of it not being so convenient at all times as that of the winch, which is, besides, much more compact, and requires for its use no previous initiation. The ordinary kind of small hand-mills resemble closely those metallic coffee-grinders which almost every person has in his possession, or may see in constant requisition in the shops of grocers. A few words will describe the whole of this clase.—They consist of one central solid fruatrum of a cone, the outer circular surface of which is cut spirally into furrows, so as to present at the upper edges of the latter a continuous series of angular teeth. On the outside of the latter is fixed concentrically a hollow frustrum of another cone, similarly cut into grooves, and so proportioned to the former, that at one extremity the opposed grooves almost touch each other, and at the other they are so far apart as to admit the articles to be ground, whole. These concentric conical grinders are fitted up in a variety of ways. In the little box-mills the axes of the cones are usually vertical ; but in the fixed, or post-mills, the cones are horizontal . in both, they are surmounted with hoppers to convey the materials to the grinding surfaces, and the products of the trituration are received into either fixed or loose receptacles beneath. By the revolution of the inner cone, the substances are first broken in the widest part of the annular crevice, and being thus reduced in size, they gradually sink, or are forced into narrower and narrower spaces, until they emerge from the grinders in a state more or less comminuted, according to the adjusted space between the grinders, which is usually performed by a screw passing through a traverse bar, with its end bearing against one end of the revolving grinder, so as to limit the extent of ite separation or distance from the fixed grinder. Mills
constructed upon the same principle are almost universally applied to a great variety of useful purposes; and the manufacture of them is one of considerable extent in Birmingham and other places. But however valuable their applica tion generally, they are but ill adapted to the grinding of corn advantageously, because the perfection of that art consists in an exact separation of the husk, or bran and pollard, from the pure flour ; and the operation cannot be succees fully performed if the corn be much cut to pieces, which mills of the kind just described almost invariably do when they are in order, or sharp; and when they are dull from wear, the mills soon clog, if set close,—or if set open, a very wasteful quantity of flour is left upon the bran or other offal. These defects arise, in our opinion, from an erroneous mode of construction. Corn and grain generally are extremely solid compact, bodies, and when reduced to powder or meal, they occupy a much larger space than previously ; consequently, as the grinding progresses, the spaces for the reception of the comminuted matter should be proportionally enlarged; but it will be observed, that the annular crevices between the concentric cones, where the grinding takes place, are rapidly con tracted into a very acute angle. Here the clogging necessarily occurs ; and unless the grinders be set considerably wider apart, so as to let the meal pass out in an extremely coarse state, the meal, by the continued attrition (or kneading as it were) becomes converted into a pasty, blackened mass. From a series of experiments made with cones of various inclinations by the writer, evidence, conclusive to his mind, was afforded of this fact, that, in proportion as the concentric cones were reduced in their height, did the flour improve ; and, finally, when he brought the surfaces down to a perfect .flo$, the products of the grinding were, in the language of the miller, more lively, and of a better colour, than in any previous experiments. From the singularly beautiful and ingenious device of concentric conical grinders, and their compactness, it is almost to be lamented that they do not succeed better with wheat. There is, however, another defect attached to these mills, which we ought not to forbear noticing ; this consists in the spiral grooves forming a series of continuous cutting edges, which clip the grain to pieces, and cause much of the husk to be ground fine, and be inextricably mixed with the flour; whereas, the action ought to be that of simple crushing, in the first part of the operation, which flattens the husk, and permits the flour to be afterwards rubbed and scraped from its surface, without incurring much subsequent minute subdivision to the detriment of the flour.