MILLS, KILNS, MALT-HOUSES, ETC. 569 and arc named according to their several uses ; as corn, snuff, bark, colour, fulling, gun- powder, cotton, linen, oil, paper, sawing, sugar, or threshing mills, &c. Of these, corn- mills are the most important, as they are requisite to convert corn into flour, the state in which it is most fit for the food of man, and they are on that account indispensable ift all civilised countries. 1239. " Corn Mills. Few persons are ignorant that corn is gi'ound between twfl- stones placed one above the other without touching. The lower millstone is im- movable, but the upper one turns upon a spindle. These stones are usually from about 4 feet to 6 feet in diameter, and vary in thickness from 12 to 18 inches. The opposite surfaces of the two stones which are to grind the corn are not plane or flat ; but the upper one is hollowed about an inch, and the under one swells up about three fourths of an inch, so that the two millstones come nearer and nearer towards the circumference, whereby the corn that falls from the hopper has room to insinuate itself between them, as far as two thirds of the radius, which is the place where it begins to be ground, and where it makes the greatest resistance it is capable of; the space between the two stones being in that place about two fifths the thickness of a grain of corn. But as the millers have the power of raising or sinking the upper stone a little, they can proportion the distance from the lower one according as they would have the flour finer or coarser. The stone used for grinding corn was formerly brought from France, and was called burr stone ; but, latterly, stones proper for this purpose have been discovered in different parts of Britain. From a quarry near Conway, great quantities are dug every year ; and when first cut out, they are much softer, and more easily worked than when they have been exposed to the air ; even a single day makes a difference. From Abbey Craig, near Stirling, a great number of millstones are sent to various parts of the king- dom, that are considered to be in some respects superior to the French. In order to cut and grind the corn, both the upper and under millstones have channels or furrows cut in them, proceeding obliquely from the centre to the circumference ; and these furrows are each cut perpendicularly on one side and obliquely on the other, into the stone, which gives each furrow a sharp edge. In the two stones they come against one another like the edges of a pair of scissors, and cut the corn so as to make it grind the easier when it falls upon the plane between the furrows. These are cut the same way in both stones when they lie upon their backs, which makes them run crosswise to each other when the u])per stone is inverted by turning its furrowed surface towards that of the lower. When the ftn-rows become blunt and shallow by constant wear, the running stone must be taken up, and both stones new dressed with a chisel and hammer. But, by this operation being often repeated, their thicknesses, and consequently their weight, are diminished ; and it lias been observed, that, when they have lost one fouJ-th or one half of their original thickness, they produce but three fourths or one half the flour which they did when new. The circular motion of the upper millstone brings the corn out of the hopper by jerks, and causes it to recede from the centre towards the circumference, where, being quite reduced to flour, it is thrown out by the centrifugal force of the stone through a hole provided on purpose. IMillstones will commonly last from thirty to forty years. 1240. " Water-Milh. In water-mills the moving power is the momentum of the water, communicated to the wheel ; and it is used in three different waj's ; first, where the force of the water is applied from below the wheel, which is called an undershot wheel ; secondly, where the water strikes nearly against the middle of the wheel, which is called a breast wheel ; and, thirdly, where the water is applied above the wheel, which is called an overshot wheel. Of these, the first is the least, and the last by far the most, powerful." 1241. " Undershot Water-whenls in a great measure act by the impulse of flowing water and are considered to be the most ancient form of water-wheel. At first they were wheels provided with vanes or wings on the circumference, called floats (see d, in fig. 1101), and were placed in a river or running stream ; the floats at the lower part of the wheel being placed so as to dip into the stream in such a manner as to intersect the water. But, in wheels constructed thus, when the planes of the floats become perpen- dicular to the direction of the current, or nearly so, they will resist or oppose the motion from it, in proportion to the quantity of motion they have abstracted from the water of the stream. The power thus obtained will be found to be only a small ])ro])ortion of the power of the stream ; because the water easily escapes sidewise from the floats, particularly if it should be attempted to take away any considerable share of its velocity by resisting or loading the wheel, so as to make it move slowly. This being discovered, it became an obvious improvement to contract the stream to the exact size of the float- boards of the wheel, or to make a close channel into which the wheel might exactly fit. The next improvement was, to intercept the river or stream of water by a dam or obstacle, in order to make it pen ujj or accumulate, till it had risen to the greatest height which could be obtained ; and to let the water out of the dam or reservoir into the
