November 1, 1887.] 



♦ KNOW^LEDGE ♦ 



evident that if the door at a is opened, the air will pass 

 directly by the course DreU, as it will take the shortest 

 course open to it. If a is closed and h open, its couree will 

 be D6U ; if a and h are closed, and c is open, the course 

 will be DcU ; if a. b, and c are closed, and <; open, the 



course will be DdU ; if all the doors are closed, the air 

 must (neglecting the dotted line at present) take the long 

 journey DeJJ. 



In practice, of course, the arrangements are much more 

 complex, but the principle is the same. It is easy to under- 

 stand that by skilful arrangements of this kind the air 

 may be made to take any longer or shorter course that may 

 be desired, according to the plan of the pit roads and 

 workings. 



But supposing a thoroughfare is required, as continually 

 happens, through a passage that must be closed to divert 

 the ventilation — that coal has to be run through the pas- 

 sage a while the air current must pass through e, what 

 must be done 1 This problem is solved very simply. In- 

 stead of a single door as marked in the diagram, two doors 

 or " stoppings " are used at sufficient distance apart to allow 

 a tub or train of tubs to stand clear between them with 

 space to spare. The coal enters one door, this door is then 

 sluit, and it proceeds to the other. 



So far I have only taken the case of continuous passages 

 or roads, but it commonly happens that a working while in 

 progress has no outlet at the working end — is a cul de sac. 

 If the extreme distance of this from the road is but moderate 

 and the coal is not exceptionally fiery, the current of air 

 crossing the mouth of the working produces a ^tir, which, 

 together with the natural diffusive action of gases, supplies 

 sufficient ventilation ; but when there is danger of accumu- 

 lation of fire-damp in the working, a brattice is u?ed, as 

 shown by the dotted line at / in the diagram. This 

 brattice, whiuh is simply a partition of wood or of " brattice 

 cloth," efl'ects a further diversion of the current, compelling 

 it to sweep round to the limit of the working at y and clear 

 out the dangerous gases that would otherwise accumulate 

 there until, with a limited supply of air, they formed an 

 explosive mixture. These brattice -walls are made by simply 

 erecting upright posts from floor to roof, and nailing the 

 brattice- boards and cloth to them, and carefully fitting to 

 roof and floor. 



Much care, of course, is required in the working of the 



doors in a large pit where they are numerous and complex. 

 A small amount of leakage from each would, of course, 

 retard or stop the current at the extreme limits of its 

 course. 



The friction of a long journey effects considerable re- 

 tardation and practically limits the distance through which 

 the air may be forced to travel. Besides this there is 

 another limitation. As the ventilating current proceeds it 

 picks up the inflammable gas and the carbonic acid expired 

 by the miners. It may even render dangerous those parts 

 of the mine that would be otherwise safe by carrying gas 

 from dangerous localities. Therefore in extensive collieries 

 a system of splits is adopted. 



This will be understood by again referring to the diagram, 

 where, instead of all the air making the long journey 

 DeU, it may be split into four independent currents, one 

 travelling by the course D&U, the second by DcU, the 

 third by Di/IT, and the fourth by DeU. But how can this 

 be done 1 It is simply a matter of balancing resistances. 

 Other conditions being equal, the resistance vai-ies with the 

 length of the journey ; therefore, if resistances be respectively 

 placed at b, c, and d, which shall be just equal to the addi- 

 tional resistance due to increased length of journey in 

 getting round by e, the current of air will divide itself 

 accordingly. This may be carried out by making a small 

 opening at b, a somewhat larger opening at c, and still 

 larger at d, while e remains fully open. In order that all 

 the air shall rush through the diminished opening at b, it 

 must do so at an increased velocity, and this involves in- 

 creased resistance, which varies directly with the length of 

 the journey and inversely with the sectional area of the 

 opening. This balancing of resistance requires skilful 

 management and the aid of regulators to vary the size 

 of the openings as required. In some of the great 

 collieries as much as 300,000 cubic feet of air is passed 

 per minute through all the complications of the roads and 

 workings. 



As already stated, the power for setting all this air in 

 motion is obtained by heating the air of the upcast shaft. 

 Formerly a stack was built over the pit, and this was heated 

 by a furnace on the surflice. In small shafts a fire was 

 suspended in the shaft, but these are now superseded in large 

 workings by an underground furnace connected with the 

 shaft by an upsloping flue which discharges all the heated 

 products of combustion into the upcast shaft, which is thus 

 converted into a great chimney, the contents of which may 

 be heated to 80° or 100° above the air in the downcast 

 shaft; l-tO" to 160° being temperatures commonly obtained 

 in the upcast shaft. 



The furnace used for heating is one with a large hearth 

 and thin fire, frequently fed with small coal ; the width of 

 the fire may be .5 to 10 feet, and the fire-bars as much as 

 6 feet long. Thus a large area of air is heated at once. 



A few figures will show the power that may thus be 

 obtained in a deep mine such as that of Ashton Moss, 

 which has a depth of 2,850 feet. Let us suppo.«e the 

 diameter of the shafts to be 12 feet each ; their sectional 

 area will be about 113 square feet, and thus each will 

 contain 113 cubic feet of air for every foot of depth. A 

 cubic foot of air at 60° weighs 1-29 oz. avoirdupois; .-. 113 

 cubic feet weigh 92 lbs., and the total column of air at 

 60°= 2,850 X 9-2, which, in round numbers, amounts to 

 26,000 lbs. But in heating air from 60' to 150°, it expands 

 one-sixth of its bulk ; therefore the hot air in the upcast 

 shaft, in such a case, will weigh one-sixth less than the cool 

 in the downcast. This difference amounts to -l:,333 lbs. 

 Thus the vis a tergo, or force driving the air through 

 the roads and workings and up the upcast shaft, will, 

 in this case, exceed a pressure of 4,000 lbs. It would 



