Chat. XIII.
MILLS.
1045
box, which is movable upon a vertical post, from whence it receives its reciprocatingmotion : it is made of wood, and the saws are tightened within it by several screws: anynumber of saws may be introduced, and at any distance apart, according to the number ofslabs or thicknesses to be cut; on the left of the plan are shown two sets of saws at work,with four blades in each set. The saws are guided by upright iron bars, which also main-tain them in a vertical position, and they are pressed upon their work by a cast-iron box,suspended by a rope that passes over a pulley, and slides down upon the upright post ofthe vibrating frame ; a weight is attached to the other end of the rope, as shown in thesection. Water is introduced by a horizontal pipe under the ceiling of the manufactory,and runs gently into the small inclined reservoir immediately above the block, which isunder the action of the saws: after the slabs have been cut, they are carried to the otherside of the machine, where they are laid flat and polished. The polisher is put in motionby the action of the vertical post, which receives its vibration from the saw-frame by thecrank upon the main shaft : a continued supply of water is introduced with the varioussubstances made use of for polishing: sharp sand is first supplied, which produces a per-fectly flat surface, then finer sand, and afterwards Tripoli red powder, which owes its cuttingquality to the oxide of iron that it contains; an oxide of tin, called putty, completes thepolish.
Hand-mills , for grindii g, are of very great antiquity, and had a variety of forms ; one ofthe most simple kind 001 sists of a strong wooden frame, on which is mounted a face-wheel,turned on proper bearingsby a winch or handle,and which has its cogs en-gaged in the staves of atrundle or lantern, on thespindle of which the re-volving stone of the mill,contained in the case be-low, is fixed. The matterto be ground is put intothe hopper at the side,from whence it passes tothe space betwixt thefixed and revolving stone,after which it is carriedaway, by means of thelower spout or trough,into a vessel prepared toreceive it: such mills aremuch used for numerouspurposes, and are an im-provement upon the Ro-man hand-mill, which wasan inverted hollow cone,working round and upon a solid one below : when the stones are laid horizontally, thereis an equal bearing and friction over the whole.
Horse-mills are frequently used in manufactories in preference to the steam-engine, andthey are serviceable an.I convenient where the labour required is not constant. The speed
HAND MILL.
Fig. 1607.
HORSE MILL.
with which a horse moves, so as to produce the maximum mechanical effect, is stated to betwo and a half miles per hour, or 220 feet per minute, or 3| feet per second; and the actual
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