The surface of the upper stone is brought to a perfectly parallel position with respect to the other, by means of four equidistant regulating screws, acting upon a brass box h, in which the lower extremity of the spindle works; so that the slightest movement of the box, effected by the screws, makes a correspondent alteration in the position of the upper stone, enabling it to be adjusted to the lower one with the nicest precision and the greatest facility.
We shall now proceed to describe the engraving on p. 156, which represents a very compact arrangement of a mill, having four pair of stones driven by a steam-engine. The stones are placed at equal distances from the centre of a square platform, resting upon cast-iron columns, and the driving is arranged beneath the platform, and supported by a framing of iron fixed to the columns. a is the horizontal shaft driven by the steam-engine, on which is fixed the bevil-wheel b, working into another bevil-wheel c, of equal diameter, fixed upon the same vertical shaft which carries the large spur-wheel d; this spur-wheel works into four pinions e e, fixed upon the spindles ff of the upper millstones, only two of which can be seen; g g are indented pinions, for the purpose of agitating the sievesplaced over the hoppers, for preventing stones and other extraneous substances entering the hopper; ii are the upper millstones; kk the lower millstones; l l the hoppers, from which the corn descends into a swinging kind of hopper, called the shoe, which is continually shaken by a short bar of iron screwed into the upper end of the spindle, and having four prongs, which, striking the shoe from side to side, distributes the corn equally over the eye of the mill stone.
The spindles of the mill-stone are supported on the iron levers m m, which can be raised or lowered to adjust the stones, by means of regulating screws at n n; o o are screws to raise the pinions e e, and cast them out of geer. The under, or bed stones, are partly sunk into circular holes in the platform, and firmly wedged therein, and a circular case incloses each pair of stones, leaving a space of about two inches all round them; and the corn, reduced to the state of meal, is thrown, by the centrifugal force of the stones, out in all directions into the case, from whence it is conveyed to the bolting machine, which is driven by a band from the drum-wheel t. The bolting machine is not shown, as the reader will find a description of an improved one under the word Bolting-machine.
In grinding wheat, it is usually the endeavour of millers to separate all the flour from the husk without pressing it so hard as to kill it, and without deteriorating its colour by making minute greys. This they have not been enabled to effect in a desirable manner with the mills constructed on the usual plan, nor by any form of construction that has hitherto appeared. The reason is obvious; - if the stones be brought so close together as is necessary to remove the firmly adhering portions of the flour from the husk, the whole of it will be, in a great degree, killed and discoloured by the violent rubbing necessary to clean the bran; on the other hand, if the stones are kept further apart so as to grind high, much of the flour will be left in the offals and bran.
With a view of meeting these difficulties, some millers have ground their wheat at two distinct operations; they have, in the first place, set their stones wider apart, or grind high; and then, after collecting the meal, and separating the fine flour from it, have passed the remainder a second time through the stones, setting them closer than before, or grinding low. Thus have they removed the whole of the flour from the husk, and preserved the good quality of a part of it; but the waste, and loss of time and power in conveying the meal from one place to another, occasioned by these several operations, together with the difficulty of separating the flour from it in the unfinished state by the ordinary dressing machine, have been found to neutralize the advantages otherwise resulting from this mode of proceeding.
In consequence of the great size and weight of the stones usually employed, the erections and fittings up of the ordinary mill are necessarily very heavy and expensive; and, owing to the several processes of grinding, cooling, dressing, and clearing up, being distinctly conducted in situations remote from each other, a considerable waste of flour, together with much unnecessary manual labour, and waste of mechanical power, are incurred. These disadvantages, which are inseparable from the old system, are completely obviated by the patent progressive corn-mill, manufactured by Messrs. Cotterill and Hill, of walsal, in Staffordshire, from the following causes: - Instead of employing only a single pair of stones of great weight and diameter (by which a large portion of the flour that is formed near the eye of the stone has to pass, with the bran, over a greater extent of surface than is necessary, thus injuring it by superfluous rubbing, besides misemploying the motive power for the purpose), the progressive mill is provided with two pair of stones, of smaller diameter; and underneath the first pair (shown in the figure, as in the case at a) is suspended and agitated a circular sieve b, which receives the product as it falls freely from the stones, and separating that portion of the flour which is sufficiently reduced, or softened, it delivers the unfinished portion into the eye of the second pair of stones underneath, shown at c c, with their case removed, as well as one of the external shutters, which inclose the whole machine when at work.
This second pair are set closer together than the first, to complete the softening of the remainder of the meal, which, in consequence of the bulk of the flour being separated from it, will be much more easily operated upon, and, at the same time, effect a saving of power. Underneath this pair of stones is placed at d, inclosed in its case, a dressing-machine, with brushes, which receives the meal from the stones as it is ground, and separates the remaining flour, as well as the different qualities of offals. When several progressive mills are employed, the meal resulting from the second pair of stones in each mill may be advantageously conducted into one dressing-machine, common to them all. In consequence of this division of the grinding operation into two stages, and the small size of the stones employed, the meal is not heated. This, together with the important circumstances of the bulk of the flour being separated from it, in the first instance, without brushing, renders the remainder fit for dressing up as fast as it is ground.
By this arrangement, therefore, it will be readily perceived, that the original colour and strength of the flour is preserved; that all the flour is separated from the bran without any injury to the bulk of it; and that the whole process of grinding, dressing, and clearing up the offals, is one continuous operation, performed in one compact machine, without waste, and with little manual labour.
The progressive mill is made principally of iron, and so arranged and put together, that, while the nicest accuracy in its adjustments, and certainty in its operations, are insured, the stones may be taken up to be dressed, and put down again with the utmost facility and ease. Its parts are readily taken to pieces, so as to make it easy of conveyance; and in consequence of all of them coming together with metallic faces, it can be properly re-connected by the commonest workman; and from its compactness and portability, it is peculiarly adapted for exportation, as the entire mill can be packed in a strong case, and the total weight of it is very little more than the stones alone of a common mill doing the same work.