C 78

THEORY AXl) BUACTICE OF ENGINEERING.

Book II.

stands by a sort of counter, having iron anvils let into it at intervals. The pulverised oreis allowed to fall on the ground, from which it is afterwards swept up and measured intobarrows, for each of which a certain price is paid. The substitute for this method of pul-verising copper ores is the crushing-mill; this consists of two parallel cylinders of iron placednearly in contact, one of which is made to revolve, whilst the other is fixed so as only toyield to great pressure : the stones of ore thrown in from above are ground between theserollers, and a cylindrical sieve is placed beneath, which being inclined at an angle of 45°,and turning on its axis, allows the particles which have been sufficiently pulverised to passthrough its holes, whilst the larger pieces fall out at the bottom and are returned to themill. The working of this machine is attended with the suspension in the air of a greatquantity of mineral dust, often of a very suffocating nature ; when inhaled even cursorily it isfound to produce ill effects upon the lungs; the ores are wetted for the purpose oflessening the escape of this dust, and any consequent loss. A further separation of themore valuable part of the pulverised ore is effected by the process called jigging , whichconsists of keeping the whole of the mineral particles suspended in water, for a timesufficient to allow of the subsidence of the more ponderous portion; this is done by theagitation of the water in the sieve in which the broken ore is placed; the more finelypulverised part passes through the interstices of the sieve, and the heavier pieces oflarger size occupy the bottom, sufficiently separated to admit of the light and worthlessstone being removed from the top with a piece of wood. The agitation of the water wasformerly produced by hand labour, and in many instances boys are employed at thiswork : the jigger is obliged to bend forward over the water, across which he generally stridesand shakes the sieve (usually 1| or 2 feet in diameter) beneath the surface of the water;when the separation of the several portions of the mineral is judged to be effected, the sieveis lifted out and the refuse removed.

Machinery has superseded this process in a large proportion of the works: two methodsare in use, by one of which a succession of sieves are kept in motion under water, by meansof a connection with a water-wheel or steam-engine; and in the other, the water itself, inwhich a number of sieves are immersed, is kept in a state of agitation by the motion of a bodyin the centre. 'Whichever of these contrivances is adopted, the only manual operations requiredare the supply of the mineral, and the removal of the worthless portions from the surface.The inferior portions of the copper ores, from which the metalliferous particles cannotbo extracted by the methods described, is subjected to the stamping mill , as are almost allthe ores of tin ; the mineral is reduced by the action of these heavy hanwners to a finepowder, which is carried by a stream of water through the perforations, made in a set ofplates of iron surrounding the boxes in which the stamps work. A series of washings ofthis powder succeeds, the principle of which is the carrying off the lighter particles by acurrent of water of graduated power, and allowing the more ponderous to remain andsubside. The number of these washings, amounting in some tin mines to about 100 fromfirst to last, causes the employment of a large number of boys and girls. The operationscalled trunking, huddling , &c., chiefly fall to the lot of the former, together with the clear-ing out of the slime pits, in which the mineral mud is collected, and wheeling this slimefor further dressing, all of which are carried on in the open air. The more delicatemanipulations are generally intrusted to females: among these what is called /ranting insome districts, and recking or racking in others, employs a great number ; in this the girlstands at the side of a very shallow wooden frame inclined at a moderate angle, and openat the foot: at the head of this, on a ledge more or less raised, a portion of the metal-liferous mud is extended, and being divided by a light rake, a gentle stream of water isallowed to find its way through it, and to carry it gradually to the fiaine below; by askilful direction of the current, the lighter portion is carried off' at the bottom, and theheavier is thrown beneath the frame by tilting it into a vertical direction upon thepivot upon which it hangs, and throwing water with the shovel upon its surface towash off' any portion which might adhere to it. The tin ores, after these successivecleanings, are removed to the calcining furnace, and are subjected to several furtherwashings: in'some of these the girls sit within and at the lower part of a long woodentrough, and direct the gentle current of water with a light brush or feather over the sur-face of the ore.

Sampling is finally preparing and dividing the ores for sale: this division into separateparcels is done by females; the general heap, containing some hundred tons, is sur-rounded by a number of pairs of girls with handbarrows, which are filled from the edgeof the heap by a party stationed round in a regular succession, directed by a girl appointedto the post; the barrows are then carried off rapidly, as the germs of a certain number ofdistinct parcels, and to each of these a barrow-full is added in regular order, so that thetotal number in every one is the same: those who fill the barrows exchange places after atime with those who carry them; the latter have during their turn by far the harder work,the barrows usually containing about 1^ cwt.

Ikon. — The ores of iron are extensively diffused throughout the mineral kingdom in