MILLS are, in the original sense, estab lishments for grinding grain, ctrc., but the word is often synonymous with factory or manufactory, and applied to any thing wrought by machinery.
Flour nulls have, for ages, been wrought by water and wind, but latterly steam has been applied. The principle is the same in all, the rotation of a stone in jux taposition with a fixed one, with hopper for supply, and bolter for sifting. Doubts being entertained of the integrity of many millers, various mills for domestic grind ing have been invented, as Stoekdale's and Rustall's, with detached bolters. Rustall's consists of vertical stones of 30 inches, one of which is turned by a winch, and altogether most complete, and his bolter is equally excellent. It has been adopted in prisons, workhouses, barracks, and other establishments.
The grain falls from the hopper into the eye of the upper mill-stone, and by its revolution is passed between the stones, and thrown out, as flour, meal, or malt. The number of revolutions in a minute ought to be 450, divided by the diameter in feet.
An upper stone, of six feet, contains about 221 cubic feet, at 840 or 850 lbs. to the foot, and its revolutions about 75 per minute. The water-wheel maybe 18 feet, if the floats move with a third of the ve locity of the stream.
A sack of wheat per hour may be ground with a power able to raise 900 lbs. 70 feet per minute ; for 2, 3, or 4 sacks, the power rises in less ratio than the multiple, i. e. 2100 lbs. at 70 feet, would grind three sacks of rye or wheat.
To grind 3 sacks per hour requires a cylinder of a steam-engine of 20 inches, and 1 sack a cylinder of 12•5 inches. To do the same work, 3 and 1 sack, with an overshot water-wheel of 12 feet, requires 1600 and 660 imperial gallons of water ; with a wheel of 16 feet, 1200 and 500 im perial gallons ; or with a wheel of 24 feet, 800 gallons and 330 per minute.
In a corn-mill, there are three depart ments. 1. The revolving sieve and fan, to clean the corn before it descends to the hopper. 2. The hopper and the stones. 3. The cooler and vibrating bol ter, where it is dressed. A system of buckets raises thegrain to the sieve, and the flour to the bolter, by various appli cations of the moving power.
Bigelow proposes to make boles in the cap or to prevent the heating or clogging of the meal, in cases of increased velocity.
Mill-stones are hard grit or granite stones, cut into grooves in octagonal sec tions, so as to cross each other in work ing, and make the grain pass over the largest surface. Eight radii, at 45°, are first grooved with a double hammer, pick, and chisel, and then six other roads or grooves are chisselled and picked parallel to each of these radii ; the stones then work parallel. Some work right-handed, others left-handed, but the grooves must tally.
Pax mills are water-mills, which turn rollers for bruising the rough flax, and revolving or turning spikes or scutchers to clear the flax presented to them. For ty aws for the water-wheel, and 102 cogs for the power, moving another with. 25 teeth for tne middle roller, are the pro portions of Gray.
Flatting mills for reducing metal to plates, are rollers turned by horses, or steam-power, through which the metal is passed and repassed. The distances of the rollers are adjusted by screws, turned by pinions. Lead, copper, and silver are rolled thus, and copper is silvered by rol ling a thin plate of silver fixed to a thick one of copper, and, by repeatedly passing through the rollers, constantly narrower or closer, a perfect thin sheet of silvered copper is produced which may be cut or stamped for plated ware, or other pur poses.
A sugar mill is formed by three upright rollers, the one moving, and the side ones fixed, or sometimes the three mov ing. The rollers are turned by mules, water, or steam-power, and the canes are placed between the rollers by hand. The expressed juice runs into a trough, and thence into a The following calculations may be use fully given under the head of mill opera tions: In an undershot waterwheel, the i quantity is the velocity in a second into the area of the water-way. And the power is the quantity into the weight of a cubic inch of water, and the velocity. The power is to the effect produced, as 10 to 8•62.