Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/90

70 COAL [MINING. 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. The endless rope system overhead is substantially similar to the endless chain. The waggons are attached at intervals by short lengths of chain 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 ventilation, 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 Avhich is kept continu ally 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 horizon tal 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 down cast, and E the upcast, and in winter, E the downcast, and D the upcast. These conditions are induced by the tern- Fig. 16. perature of the earth at a certain depth being nearly con stant, 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. The former plan, although hitherto most generally used, is in many places becoming replaced by some form of machine. Furnace. The usual form of ventilating furnace is a plain fire grate 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 G to 10 feet broad and from G to 8 feet long are usual dimensions. At Shircoaks Colliery, in Nottinghamshire, a furnace con suming G 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 Iletton 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 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 con tact 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. Various kinds of machines for ventilation, both by direct M exhaustion and centrifugal displacement, have been tried ven both in England and in Belgium. Of the former class are the great bell machines, resembling gasometers, 12 feet to 22 feet in diameter, 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 Li6ge, and at Cwm Avon in South Wales, by Mr Struve". 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 hori zontally 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 conse quently 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 Hoot 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 cylin drical 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 tbe 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. 552, 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 Lie ge, 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, BO as to 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