in this country, is represented in Fig. 624, in which the letters have the following meanings: G, hopper, into which the ore is shovelled from the floor, HH; A B, the two cylinders or rolls shown on a larger scale in Fig. 625. The roll В has plummerblocks which can slide along a bed-plate, and so allow the opening between it and the roll A to be increased or diminished; C is a bent lever, to one end of which is attached a weighted box, whilst the other constantly presses a pin against the plummer-block of B; the crushed rock after leaving the rolls falls into a revolving cylindrical sieve. All that fails to pass through the sieve drops into the "raff-wheel" E, which has buckets on the FIG. 624.

face turned towards the crusher; these carry up the coarse fragments as the wheel revolves and tip them on to a sloping apron F, whence they fall again into the hopper G to undergo a further crushing.*

One end of the shaft of the roll A is coupled to the main driving shaft of the machine, which carries the raff-wheel; the other end has a cog-wheel which gears into a similar one on the shaft of B, and so drives it. The inclined sieve is driven from the shaft of A by means of bevel gearing.

An improved form of the Cornish rolls has been introduced by Krom,† and is meeting with approval. His improvements are:

* Ferguson, "On the Mechanical Appliances used for Dressing Tin and Copper Ores in Cornwall," Proc. Inst. Mech. Eng., 1873, plate liv., and p. 133.

+Krom, "Improvements in Ore-crushing Machinery," Trans. Amer. Inst. M. E., vol. xiv., 1885, p. 497.

Steel tires, pulley gearing, housing to enclose the rollers, swinging pillow-blocks, tie-bolts to take the crushing strain, hopper for automatically ensuring a regular feed.

FIG. 626.

STEEL TIRE

(ON BONES

STEEL SHAFT

The tires (Fig. 626) are made of mild forged steel,* and are held by two cores in the form of truncated cones. One of the cores is shrunk firmly on to the main shaft, the other is split on one side, but when drawn in towards its fellow by bolts, it grips the shaft very tightly, and at the same time fastens the tire securely. The main shaft (Fig. 627) † is driven by a pulley, indicated by the dotted line, revolving at the rate of 80 to 100 times

FIG. 627.

10

SCALE

އ

Messrs Bowes Scott and Western use a special steel of their own

which is said to be exceedingly durable.

+ Curtis, op. cit.

a minute; the other shaft is driven at the same speed, but in the opposite direction, by crossing the driving belt of the smaller pulley. The bearing of the shaft of the movable roll is carried by a swinging pillow-block pivoted underneath, and constantly drawn towards the other roll by the strong spiral springs. The upper part of the figure represents the bottom of the hopper which supplies the rolls, and the oscillating feed-tray, set in motion by an excentric.

Actual experience extended over a considerable time has proved that a pair of Krom rolls at the Bertrand Mill, in Nevada, will crush 150 tons of quartzose silver ore in 24 hours, so as to pass through a screen with 16 holes to the linear inch. It is claimed that less fine dust is produced with these rolls than with stamps, a matter of importance, owing to the losses in dressing or lixiviation when there is a large proportion of slime.

Fluted rolls are used in crushing rock-salt, and toothed rolls are used for breaking comparatively soft minerals such as rocksalt or gypsum, and even hard stone for road-metal. Some of the rolls for rock-salt are made of toothed rings threaded upon a shaft, and the two rolls are arranged so that the teeth of one lie between those of the other.

d. Mills-The term "mill" has a very vague signification among miners; all sorts of machines employed in crushing and grinding are commonly known as mills. I propose to restrict the term to grinders, in which the working parts consist of flat or approximately flat surfaces, one of which revolves. They are called into requisition for reducing a mineral to a fine state of division.

The typical mill of this class is the well-known flour mill, made of two horizontal cylindrical stones, one fixed, the other revolving; sometimes it is the lower stone that is fixed, sometimes the upper. Mills of this kind serve to grind barytes and fertilisers. The stones are generally the French burr, and have to be dressed from time to time as they wear. The mineral is fed in at the centre, and is discharged at the circumference. Instead of one top stone, there may be several separate pieces; this combination forms the "arrastra " employed for grinding and amalgamation.

When the mill is made of iron, with iron or steel replaceable wearing parts, it is generally called a "pan"; like the arrastra, it serves for fine grin ling and amalgamating.

Millstones need not necessarily be arranged horizontally; the first grinding of phosphate of lime is sometimes done by stones set vertically, the moving stone being fixed upon a horizontal

axis.

e. Edge-runners.-The edge-runner is a cylinder turning upon a horizontal axis which is made to revolve around a vertical axis. In its simplest form, it is a large stone wheel, the horizontal axis

of which is drawn round an upright post by a mule. The stone crushes by its weight, and as it has to slide a little in order to keep its circular path upon the bed, there is also a rubbing action. This primitive form of edge-runner, known as the Chilian mill, is employed in crushing and amalgamating gold and silver ores. It is better to have two of the upright wheels at opposite ends of the horizontal axis, as then the machine will work more smoothly (Fig. 628). Each wheel is made of a strong tire of chilled cast-iron wedged to a centre-piece of ordinary cast-iron, and the bed is composed of sectors of chilled cast-iron, which can

be changed when they are worn. The driving gear may be above or below.

f. Ball-grinders.-In machines of this class the mineral is pulverised by its contact with a number of cast-iron balls, which are constantly rolling against each other when the case containing them revolves.

Jordan's Centrifugal Grinder and Amalgamator is a circular pan set upon an inclined axis with a few large iron balls like cannon-balls which lie in the lowest part; the machine is supplied with crushed ore, which is soon ground fine and escapes through a sieve placed around the outside of the pan.

The ingenious "Grusonwerk" ball-grinder (Figs. 629 and 630), now made by Krupp, has a continuous feed and discharge. It consists of a horizontal iron cylinder provided with several curved plates

FIG. 629.

FIG. 630.

through holes in the curved plates a a, and in the cylindrical sieve c made of punched steel plate; it now meets with the fine wire gauze sieve d, which lets through all that is sufficiently pulverised into the hopper s, whence it can be drawn off at pleasure. The object of the punched steel sieve c is to prevent the unnecessary wear of the fine wire gauze, which would naturally suffer if it

a a, which carry a number of steel balls. The stuff which is fed in by the hopper h falls among the balls and is ground by their rubbing. During each revolution of the drum, they drop five times as they come to the edges of the plates. The ground mineral passes