664

THEORY AND PRACTICE OF ENGINEERING.

Book II.

they arc forced to recede sufficiently to let it pass through, when other pieces of stoneslip through unbroken, although but for this contrivance the mill might be damagedor choked up. The rollers are, however, immediately brought back to their right positionby means of levers, at the end of which are attached large weights, usually stones, foreffecting this purpose, and the depression of the lever brings up the roller attached to itto its proper position.

From the pits below the crushing-mill the broken ore is drawn up to the chat mill,on the right side, by means of iron buckets on an endless chain, much in the same way aswe see, on a larger scale, the ballast dragged up into the barges from the bottom of theriver Thames . Every bucket, on arriving at the top, discharges its load upon a grating,by the bars of which the larger pieces are retained, and are passed again through thecrushing mill, or sent to the stamping mill; the smaller pieces are made to pass thethree pair of chat rolls, which are exactly on the same plan with the crushing rolls, onlyon a smaller scale, and adapted to ore of a smaller size. As the crushed ore comes downfrom the chat mill, a boy stirs it, and the small lead, with the dirt adhering to it, is carriedby the stream of water to pits lower down.

'The stamping mill is used for breaking the hard refractory pieces of ore, which resist therollers of the crushing and chat mills. In some establishments the stampers are separateand distinct from the crushing-mill, and in others the same water-wheel turns the rollers ofthe crushing-mill, and raises the stampers, the broken ore being carried down an inclinedplane by a stream of water. When the matrix of an ore is soft and easily broken, thestamping mill may be dispensed with, but for very hard ore it is exceedingly useful.

After the ore has come from the chat mill and the smaller portion has been carried offby water, it is taken up and put into a sieve, to undergo the process called hutching. Thesieve, made of iron wire, is let into a box which is full of water. From the stalks or chainsof the sieve proceed a long lever which rests upon a fulcrum ; this is moved up and downby a boy, who places his two hands above his head, and pulls the end of the lever, and inconsequence the sieve with the ore upon it is raised up and down with an agitated motionin the water contained in the box, which occasions the very small lead or dust, calledSmiddum, to fall through the sieve, and sink to the bottom of the box ; and of that portionwhich remains above the sieve, the lead, being the heaviest, works down to the lowest placenext the wires of the sieve. Immediately above the lead are the larger pieces of stone,with portions of ore called chats, and above the chats are lighter pieces of stone calledcuttings: the cuttings are removed by a limp or broad piece of iron, and given to thecutting cleaners, when it is again put into a sieve and treated as before ; the chats arcsent back to the mill to be ground again.

It has been already stated that when the ore is laid on the grating, the smaller portionsare carried through the bars to a pit below ; by a stream of water, part of this matteris sludge or slime, but there is another portion much too large and weighty to bethus carried off: this is taken out of the pits, again put in the sieves, and hutched orjerked up and down in the water on the sieve, by the boy pulling the end of the lever;and when sufficiently hutched, the stony matter is carried off by the limp, and the clean orelying at the bottom is taken to the bingstead.

The Smiddum is taken from the bottom of the boxes, in which the sieves were agitated,and removed to a running buddle, or space of ground with a stone floor made a little lowerthan the ground about it, and having a little declivity, over which water runs very gently;upon the upper end of this buddle is put the smiddum, and the water let in upon it.The boys and young persons then stir it with an instrument called a colrake , and the watercarries away dirt, and the fragments of stone or cuttings, and the lighter ore or smiddumtails are brought to the lower end of the buddle, whilst the weightier ore is left at the upper.The two are thus separated, and the weightier ore is removed to the bingstead.

It is a necessary consequence of the grating and crushing of the ore under the action ofwater, that a quantity of finely pulverised earthy matter has been collected, and much lead,in the form of minute detached particles, been brought away with it, carried down thestream with the water, and lodged in the pits into which it flows, where all this matteris merely mechanically diffused through it. This mass is more or less stiff, and that portionwhich is coarse and contains larger grains of lead, is called sludge, that consisting of smallerand finer particles slime : it is put into trunks and again agitated with water, then laid onthe floors of the buddies, and streams of water made to pass over and through it, beingstirred and rubbed against the bottom of the buddies whilst the water is flowing, the objectbeing to separate the lead from the clayey matter. The last process is to put the slimeinto the dolly-tub, where by means of a handle the board is turned round which agitatesthe slime; the lead then falls to the bottom, and the other matter above is taken away.Many particles of the lead are. however, carried down in the muddy water of the river orburn, and the cattle for many miles below a washing place are not allowed to drink of it.

The apparatus of the Houghton Gill Mine, invented by Mr. John Leathart, of Alston,in Cumberland, and put up in 18*10, is found to be of use in facilitating these operations,