rails to prevent them from chafing and wearing out on the ground. Where the load has to be hauled up a rising gradient, underground engines, driven by steam or compressed air, are now generally used. In some cases steam generated in boilers at the surface is carried in pipes to the engines below, but this can be done with less loss of power by sending down compressed air in the same way. The use of underground boilers placed near the upcast pit, as in fig. 6, so that the smoke and gases help the ventilating furnace, is most convenient in the majority of cases. Water-pressure engines, driven by a column of water equal to the depth of the pit, have also been employed for hauling. These can, however, only be used advantageously where there are fixed pumps, the fall of water generating the power resulting in a load to be removed by the expenditure of an equivalent amount of power in the pumping engine above that necessary for keeping down the mine

water.

There are four principal methods in which steam power can be applied to underground traction. These, which have been discussed in the fullest manner in the Report of the North of England Institute of Mining Engineers for 1867-68, are as follows:

1. Tail rope system.

2. Endless chain system.

3. Endless rope system on the ground.

4. Endless rope system overhead.

The three last may be considered as modifications of the same principle. In the first, which is that generally used in Northumberland and Durham, a single line of rails is used, the loaded tubs being drawn "out bye," i.e., towards the shaft, and the empty ones returned "in bye," or towards the working faces, by reversing the engine; while in the other systems, double lines, with the rope travelling continuously in the same direction, are the rule. On the tail rope plan the engine has two drums worked by spur gearing, which can be connected with, or cast loose from, the driving shaft at pleasure. The main rope, which draws out the loaded tubs, coils upon one drum, and passes near the floor over guide sheaves placed about 20 feet apart. The tail rope, which is of lighter section than the main one, is coiled on the second drum, passes over similar guide sheaves placed near the roof or side of the gallery round a pulley at the bottom of the plane, and is fixed to the end of the train or set of tubs. When the load is being drawn out, the engine pulls directly on the main rope, coiling it on to its own drum, while the tail drum runs loose paying out its rope, a slight brake pressure being used to prevent its running out too fast. When the set arrives out bye, the main rope will be wound up, and the tail rope pass out from the drum to the end and back, i.e., twice the length of the way; the set is returned in bye, by reversing the engine, casting loose the main, and coupling up the tail drum, so that the tail rope is wound up, and the main rope paid out. This method, which is the oldest, having been in use for twenty-five years or more in the North of England, is best adapted for ways that are nearly level, or when many branches are intended to be worked from one engine, and can be carried round curves of small radius without deranging the trains; but as it is intermittent in action, considerable engine-power is required in order to get up the required speed, which is from 8 to 10 miles per hour. From 8 to 10 tubs are usually drawn in a set, the ways being often from 2000 to 3000 yards long. In dip workings the tail rope is often made to work a pump connected with the bottom pulley, which forces the water back to the cistern of the main pumping engine in the pit.

For the endless chain system, which is much used in the Wigan district, a double line of way is necessary, one line for full and the other for empty tubs. The chain passes over a pulley driven by the engine, placed at such a height as to allow it to rest upon the tops of the tubs, and round a similar pulley at the far end of the plane. The forward edge of the tub carries a projecting pin or horn, with a notch into which the chain falls which drags the tub forward. The road at the outer end is made of a less slope than the chain, so that on arrival the tub is lowered, clears the pin, and so becomes detached from the chain. The tubs are placed on at intervals of about 20 yards, the chain moving continuously at a speed of from 2 to 4 miles per hour. This system presents the greatest advantages in point of economy of driving power, especially where the gradients are variable, but is expensive in first cost, and is not

VOL. VI.-239

Fowler's clip pulley may be used. It is also customary to use a stretching pulley to keep the rope strained when the pull of the load diminishes. This is done by passing a loop at the upper end round a pulley mounted in a travelling frame, to which is attached a weight of about 15 cwt. hanging by a chain. This weight pulls directly against the rope; so if the latter slacks, the weight pulls out the pulley frame and tightens it up again. The tubs are usually formed into sets of from 2 to 12, the front one being coupled up by a short length of chain to a clamping hook formed of two jaws moulded to the curve of the rope which are attached by the "run rider," as the driver accompanying the train is called. This system in many respects resembles the tail rope, but has the advantage of working with one-third less length of rope for the same length of way.

Ventila

The endless rope system overhead is substantially similar to the endless chain. The wagons are attached at intervals by short lengths of chains lapped twice round the rope and hooked into one of the links, or in some cases the chains are hooked into hempen loops on the main rope. One of the most important branches of colliery work is the management of the ventila- tion. tion, involving as it does the supply of fresh air to the men working in the pit, as well as the removal of inflammable gases that may be given off by the coal. This is effected by carrying through the workings a large volume of air which is kept continually moving in the same direction, descending from the surface by one or more pits known as intake or downcast pits, and leaving the mine by a return or upcast pit. Such a circulation of air can only be effected by mechanical means when the

workings are of any extent, as will be apparent from the following considerations:

If the shafts A and B, fig. 15, were of equal depth from the horizontal plane, and connected by the mine C, the air would fill the openings and remain quiescent. If the one were to the dip of the other, but communicating with the surface at a higher level, as by fig. 16, it would sometimes happen, in summer, that D would be the downcast, and E the upcast, and in winter, E the downcast, and D the upcast. These conditions are induced by the temperature of

the earth at a certain depth being nearly constant, while the atmosphere is changeable, the column of air in D d being at a lower temperature in summer than the column of air E e, and the reverse in winter.

The methods actually adopted are-(1) The rarefaction of the air in the upcast pit by a furnace placed at the bottom; and (2) Exhaustion by machinery at the surface.

FIG. 17.-Guibal's Fan.

The former plan, although hitherto most generally used, is in many places becoming replaced by some form of machine.

The usual form of ventilating furnace is a plain Furnace. firegrate placed under an arch, and communicating with the upcast shaft by an inclined drift. It is separated from the coal by a narrow passage walled and arched in brick-work on both sides. The size of the grate varies with the requirements of the ventilation, but from 6 to 10 feet broad and from 6 to 8 feet long are usual dimensions. At Shireoaks Colliery, in Nottinghamshire, a furnace consuming 6 tons of slack per 24 hours upon a grate surface of 72 square feet maintains a circulation of about 120,000 cubic feet per minute. At Hetton Colliery, Durham, the grate is a long, narrow rectangle, 25 feet by 5 feet, with numerous furnace-doors on the long side, so arranged that the surface fired may be varied according to the amount of draught required. There are two bunker-holes for coals, and a stoking passage, 7 feet wide, in front of the furnace. The fire should be kept as thin and bright as possible, to reduce the amount of smoke in the upcast. When the mine is free from gas, the furnace may be worked by the return air, but it is better to take fresh air directly from the downcast by a scale, or split, from the main current. The return air from fiery workings is never allowed to

FIG. 18.-Waddle's Fan.

approach the furnace, but is carried into the upcast by a special channel, called a dumb drift, some distance above the furnace drift, so as not to come in contact with the products of combustion until they have been cooled below the igniting point of fire-damp. Where the upcast pit is used for drawing coal, it is usual to discharge the smoke and gases through a short lateral drift near the surface into a tall chimney, so as to keep the pit-top as clear as possible for working. Otherwise the chimney is built directly over the mouth of the pit.

ventilation.

Various kinds of machines for ventilation, both by direct exhaustion and centrifugal dis- Mechanical placement, have been tried both in England and in Belgium. Of the former class are the great bell machines, resembling gasometers, 12 feet to 22 feet in di ameter, and 9 feet high, moving in a water tank with balanced flap valves for alternately admitting and exhausting the air. These were used at Marihaye, near Liége, and at Cwm Avon in South Wales, by Mr. Struvé. Perhaps the largest of the class of piston machines is that at Nixon's Navigation Pit, near Aberdare, which has rectangular pistons, 30 feet by 22 feet, moving hor izontally through a stroke of 7 feet, the lower edge being supported by rollers running on rails. The great weight of the moving parts in this class of machine makes them incapable of acting at any very high speed, and consequently expensive for the amount of work done. This is in some degree obviated in the rotary piston machines of Fabry and Lemielle, the former resembling in principle Root's blower, now so much used in blowing foundry and smiths' fires, but on a larger scale. Lemielle's ventilator is a vertical drum revolving eccentrically within a cylindrical casing. The drum carries three jointed blades, which are drawn in or out by radius bars as it revolves, so as to enclose and sweep out at each revolution the body of air included between the two cylinders. This is one of the best machines of its class, producing a comparatively high effect for the power expended. An American machine of this kind is described and figured in the article BELLOWS, vol. iii. p. 476, fig. 5.

Of late years various kinds of centrifugal machines, or fans, have come into use instead of ventilating furnaces. One of the most successful of these is that invented by Mr. Guibal of Liège, represented in fig. 17. The fan has eight arms, framed together of wrought-iron bars, with diagonal struts, so as to obtain rigidity with comparative lightness, carrying flat close-boarded blades at their extremities. It revolves with the smallest possible clearance in a chamber of masonry, one of the side walls being perforated by a large round hole, through which the air from the mine is admitted to the centre of the fan. The lower quadrant of the casing is enlarged spirally, so as leave a narrow rectangular opening at the bottom, through which the air is discharged into a chimney of gradually increasing section carried to a height of about 25 feet. The size of the discharge aperture can be varied by means of a flexible wooden shutter sliding in a groove in a cast-iron plate, curved to the slope of the casing. By the use of the spiral guide casing and the chimney, the velocity of the effluent air is gradually reduced up to the point of final discharge into the atmosphere, whereby a greater useful effect is realized than is the case

when the air streams freely from the circumference with a velocity equal to that of the rotating fan. The power is applied by steam acting directly on a crank at one end of the axle. In most of the newer examples, which are generally of large size, the power is divided, an engine being placed on each side. At Washington Colliery, Durham, a machine of 36 feet diameter, 12 feet breadth of face, and 13 feet diameter of intake passage, draws 120,000 cubic feet of air per minute, when making 38 revolutions. Another at Usworth, 48 feet diameter and 12 feet breadth of face, driven by two high-pressure engines, with cylinders 3 feet in diameter and 3 feet stroke, equal to about 280 horse-power, exhausts 200,000 cubic feet per minute. The useful effect realized under the most favorable conditions is as much as 50 per cent. of that of the steam power employed.

Waddle's fan, represented in fig. 18, is an example of another class of centrifugal ventilator, in which a close casing is not used, the air exhausted being discharged from the circumference directly into the atmosphere. It consists of a hollow sheet-iron drum formed by two conoidal tubes, united together by numerous guide blades, dividing it up into a series of rectangular tubes of diminishing section, attached to a horizontal axle by cast-iron bosses and wrought-iron arms. The tubes at their smallest part are connected to a cast-iron ring, 10 feet in diameter, but at their outer circumference they are only 2 feet apart. The extreme diameter is 25 feet. A fan of these dimensions at Brownhills in Staffordshire, in making 50 revolutions