shafts through which the lubricant works under the hub of the pulley. Bottom idlers, dd, are employed for supporting the returning empty side of the belt, and sometimes guide idlers are used which perform no function ordinarily, but only come into use should the belt run sideways from settling of the framework or other cause.

There are only two component parts, a set of fixed pulleys and a belt, while the conveying portion is entirely separate from the running portion. The material is received directly on the troughed belt and never comes into contact with the pulleys to retard or clog their action, while the point where the load is received is the only point of abrasion or friction between the material and the belt. Experience having shown that the wear is greatest in a line along the centre of the belt, the rubber-protecting cover is made of extra thickness there, while the belt is further stiffened by running two or three plies of duck a part of the way in from the edges.

Swinging conveyors made under the Kreiss-Zimmer patents have come into considerable favour owing to their simplicity, large capacity, and small amount of driving power required. They consist of a trough, a, Fig. 585a, fixed on inclined spring legs, b, securely bolted to the floor or other support. The trough receives a swinging motion from a countershaft and crank through the rod, c, which is not rigidly fixed to the trough, but has a strong spring placed on each side of the attachment. The combination of the reciprocating motion from the

crank and the rocking of the flexible legs, causes the material which is deposited at one end of the trough to travel with great rapidity, and yet in a gentle floating manner to the other end. When the trough is full, the material seems to move in a solid mass, and there is little friction between it and the trough owing to the hopping motion which takes place.

For long lengths, such conveyors are balanced by dividing them into two equal portions, and arranging one a little above the other, so that the higher trough can deliver into the lower. The driving crank is made double, one part fixed 180° in front of the other, so that one-half of the conveyor moves forwards while the other half moves backwards, but as the spring legs are fixed at the same inclination, the material travels in the same direction in both sections of the conveyor. Their chief merit is their simplicity, as there are only two bearings and two cranks to keep in order and oil whatever the length may be. Material can be withdrawn at any number of points by providing ordinary slide doors in the bottom of the trough. They can be used as picking belts in combination with any screen, and, indeed, by perforating the bottom of the trough, dust may be separated from the larger coal as the material passes along. These conveyors will carry material up an incline, but their capacity diminishes rapidly as the gradient increases.

Coating Steam Pipes.-To prevent condensation and loss of heat by radiation, boiler houses are often roofed in, not perhaps so often at collieries as they should be. The pipes conveying steam to the different engines should also be protected by some external covering, not only to minimise radiation, but to prevent the severe strains which would otherwise be set up from expansion and contraction if such were not done. Such coverings are more necessary than ever now that the steam pressures are being increased. A very good cheap kind is to bind on a series of rough wood bars all the way round the pipes. At Mariemont, the covering employed possesses the advantage of being movable. A zinc or sheet iron-tube surrounds the steam pipe, and a layer of non-conducting material is placed between (Fig. 586). These tubes are made in halves with a hinge, a, at one side, and a clasp, b, at the other.

The ordinary practice is to put a series of layers of composition along each pipe and to let the first coat dry before the next is added, and repeat the process until the material is sufficiently thick. The covering does not extend from flange to flange of each pipe, but is tapered away at each end in order that the bolts may be unscrewed and a joint remade without breaking any of the composition off. The result is that the pipe is uncovered at the flanges and for a few inches on each side, and a great deal of condensation takes place there. It is consequently advisable to use a movable covering

Pipe

at such places similar to that shown in Fig. 586. Not only does this save steam, but it reduces the strain on the flanges from expansion.

A great many different kinds of non-conducting materials for covering steam pipes are in existence. The subject has been most carefully gone into by the late Mr. W. J. Bird, who states that in an actual case the loss of steam Fig. 586. was reduced from 12.16 per cent. when the pipes were uncovered to 1.86 per cent. with covered pipes. The saving is increased by increasing the thickness of the covering, but this thickness has an economical limit. It may broadly be stated that the great majority of compositions give very satisfactory results, and that the worst of them is better than nothing at all. They are liable to deterioration from damp and heat, and should be protected by a covering of tar; in places where the covering is liable to receive blows it is further protected by a layer of felt, followed on the outside by a sheeting of zinc.

Workshops.-As the great majority of collieries are situated away from towns, it is very necessary that they should be provided with mechanics' shops, either of a simple or elaborate character, depending on the size of the mine. At the largest collieries nearly everything is made on the ground, indeed, in many cases, new engines are built there, and the shops rival those of engineering establishments. At all mines a certain staff of mechanics have to be retained to attend to breakages, and if good men are to be kept, they must have regular employment. It is far better to do repairs on the spot than to send them away. Not only is time saved, but the cost is reduced, as urgency work has always to be paid for at increased rates. In all cases a small lathe and drilling machine should be put down; in

the smith's shop, the fires should be blown by fans, and a steam hammer erected, this tool being perhaps the most useful about any colliery.

The practice of building and repairing railway waggons at the mine is now becoming common, and elaborate wood-working machinery is put down for the purpose. Boring and morticing machines and band saws are then required, but in all cases the introduction of woodboring machines results in economy. If performed by hand, the operation is a most laborious one, while a small machine with revolving auger can be purchased very cheaply.

For sawing timber, either for sleepers or for props and bars, circular saws are invariably put down. For cross cutting, the saw is carried on an iron swing frame, suspended from beams overhead and drawn against the piece of timber placed in front. The pendulum frame is counterbalanced to move back after the cut has been made. Such an apparatus works satisfactorily with all ordinary sizes, but for the larger logs, the arrangement shown in Fig. 587 is adopted.

The spindle and saw are mounted on a travelling carriage which slides on the cast-iron bed of the machine. Motion is given to the beltdriven pulley, a, fixed on the shaft, b, in which a long key-way is cut, and through bevel gearing a movement of rotation is given to the circular saw, c. The back and forward movement of the saw can be obtained by means of a screw driven by gearing from the main shaft, or it can be moved to and fro by an arrangement of levers, and when pulled forward by the attendant in the direction shown by the arrow, cuts through the timber placed before it. The pulley, a, remains in its place, but the key slides in the long key-way, and the shaft, b, continues revolving.

Bibliography.-The following is a list of the more important memoirs dealing with the subject matter of this chapter:

So. WALES INST.: Feed Water Heaters, A. C. Elliot, xviii., 370.

BRIT. SOC. MIN. STUD.: Meldrum's Patent Dust Fuel Furnace, W. H. Mungall, xiv., 165.

FED. INST. Steam Boilers with Forced Blast, Bryan Donkin, iv., 154; Powdered Coal for Firing Steam Boilers, Bryan Donkin, xi., 321.

501

CHAPTER XIII.

PREPARATION OF COAL FOR MARKET.

General Considerations.-No operation connected with mining has passed through greater changes during the past few years than that of cleaning and sorting the coal. In this country so many good coals existed that a ready sale was found for them in the state they came from the mine. Naturally the best seams were worked first, but as they became exhausted, the inferior qualities had to be mined. It, therefore, became necessary not only to adopt a more equal division into sizes, but to employ some means for removing the impurities, in order that the dirty coal in its clean state may become equal, if possible, to the good coal in its dirty condition.

The trade of the present day requires a more careful division into sizes than it did a few years ago, and for such reason the means employed to obtain such division have become much more elaborate and made to perform their work more accurately. The coal coming out of the mine has first to be emptied on to a screen, an operation which is performed by various machines called "tipplers." After passing over and through the screen, the mineral is received on travelling bands or belts, and the dirt picked out of it by attendants. It passes from the belts into shoots and thence to waggons, in which it leaves the colliery.

So long as the coal is large, the stones and dirt mixed with it can be picked out fairly easily, but in the smaller qualities where the refuse is fine, other means have to be adopted, either dry or wet cleaning, called "washing." Both methods depend on the different specific gravities of dirt and coal. In the former, a current of air is directed on to the mixed material, and blows the lighter coal farther than the heavier refuse. In the latter, moving water is employed which has the same effect. The former has not received a very extended application, but the latter is not only largely employed, but is becoming more and more used every day.

Although it would be impossible to give here anything like a complete description of the many varied types of installations which are carried out in different countries to suit different conditions, yet the main features of the various parts of the apparatus used for coal cleaning will be considered under their respective heads, and an outline given of the way several plants are arranged.

One important point must be dwelt on at the outset-viz., that it is impossible to force the trade of any district to take a certain class of coal, and that cleaning and sorting appliances must be put down at each colliery to suit the trade of that district. What is acting very well at one place with economical results might work just as economically at another place, but if the sizes and qualities made are not suited to the trade of the second district, the result of its application would be a failure. Before, therefore, adopting anything,

it is essential that the conditions under which it is working should be compared with the conditions under which it will have to work in its new situation.

Circulation of Tubs.-As soon as the tubs leave the cage at the surface, they have to be conveyed to the tipplers, and, after emptying, returned again to the pit mouth. If the screens are near the shaft, the heapstead will be covered with iron plates called flat sheets," upon which the tubs can be turned about in any direction. A better plan is to lay lines of rails and ensure movement taking place in definite paths. The tubs can then be pushed, or the rails placed at such an inclination that the tubs gravitate towards the discharging place. As they are generally taken off the cage on one side, and put on it again from the other, if it is downhill to the tipplers, it

a

Figs. 588 and 589.

must be uphill going back, and consequently a more or less greater expenditure of labour is required to perform the haulage, the amount depending on the size and weight of the tubs.

No better appliance has been introduced for minimising the cost of conveying tubs about the heapstead than that known as the "finger" or "creeper" chain, which was originally designed by a Belgian engineer. It consists of an endless chain travelling in the centre of the railway under the tubs, provided at intervals with vertical projecting pieces of iron (a a, Fig. 588 and 589) fastened to the links. The chain is built up by arranging two narrow links alternately with a broader one. These

Fig. 590.

are connected together by a pin, c, having a round head at the one end and a T-shaped head on the other, which is readily slipped through the three components of the chain, and allows of an easy extension or reduction in the length of the creeper. The projecting catches can be inserted at any desired interval. The entire length of the top half of the chain rests on a wooden baulk, b, which acts both as a support and a guide. It is driven by a sprocket or cogwheel, the teeth of which have a pitch equal to that of the chain. When a tub is conducted to the commencement of this chain, the first passing hook seizes the axle and drags on the tub, which is released at the other extremity.

Such apparatus is generally arranged as in Fig. 590. The tube