quadrant acting on a pinion attached to the pointer spindle. The helical spring surrounding the spindle is intended to take up all backlash, by keeping the pinion teeth always bearing upon one side against the quadrant teeth. To protect the steel diaphragm from the corrosive action of the steam it is covered by a thin pliable sheet of silver foil shaped to fit the corrugations. This sheet is so thin as to allow free movement of the diaphragm whilst effectively protecting it from corrosion.

LIGHTNING DESCENDING SHAFTS.

A danger to which collieries are liable is the descent of lightning to the underground workings during a thunderstorm. Well authenticated instances have been placed on record which leave no doubt that lightning has been observed to traverse the underground workings; but in all instances without serious damage having resulted from it. Some mines are so placed that they seem to invite the descent of lightning into the workings. The colliery may be situated on elevated ground, the underground workings be dry, and the ropes, rapper wires, iron guides, steam or water pipes afford the means of conducting the electric spark to the flat sheets at the landing stage, and these again being connected with the rails leave an unbroken course for the lightning, especially if the rails are "fished" at the joints. Whether it is possible for the lightning to fire an explosive mixture of air and gas in the workings is not considered certain although surely highly probable. The fact that an explosion occurred at Risca some years ago during a thunderstorm has given rise to some discussion on the subject.

To prevent the possibility of lightning discharges entering pits, the mouths of the shafts should be efficiently protected, and this may be done by fixing lightning conductors to all lofty chimneys and buildings in the neighbourhood, and these conductors should terminate with a good earth-plate sunk in the ground, or preferably in the bed of a stream or a reservoir of water. The lightning conductors should, of course, be placed sufficiently above the chimneys or buildings. If these are at some distance from the shaft it may be advisable to fix a conductor at the headstock of the pit, taking care to carry it as high as possible and to have the end carried to an earth-plate.

AUTOMATIC GREASING APPARATUS.

Many forms of tub-greasers have been designed with a view to save labour in applying the necessary lubricant to colliery tubs, and to avoid wasting the lubricant in the act of its application. Some of them do their work most inefficiently, and are liable to get out of order and damage the rolling stock they are intended to lubricate. There can be no doubt, however, that a welldesigned automatic tub-greaser is calculated to effect a considerable saving at large collieries. The following description of Dunford and Emens' patent automatic greasing apparatus (Figs. 977 and 978) is given by Messrs. Dunford Brothers, of Newcastle-on-Tyne :

"A-Semi-circular trough, fixed to the sleepers, to contain the grease, with bevelled side to incline the grease towards the wheel. B-Two side plates to carry the wheel, with slots to allow of the wheel being easily raised or lowered. C-Corrugated wheel, with 4 short arms. The wheel revolving in the grease, raises in its corrugations a sufficient quantity of grease to efficiently lubricate the axles. D-Four best tempered steel spiral springs, which hold the wheel in position, and allow of its eccentric action. E-Four cases, to hold the springs,

the short arms of the wheel being inside the springs and below the level of the top of the cases. F-Square boss, on which the spring cases slide, allowing the wheel to assume an eccentric position when struck at an angle. G-Centre piece, on which the square boss revolves, allowing the wheel to have a rotary motion; a bolt and nut passing through the centre piece holds it firm to the side plates. “The apparatus is intended to be placed in a convenient situation, one on each side between the rails of the tramway, so that each tub of a train passing over it in either direction will receive on the journals of each of its axles an adequate quantity of the grease lubricant, any excess being returned to the reservoir.

"It consists of a semi-circular trough containing the lubricant, the upper end of which is level with the ground, and covered in to keep out dirt or coal dust, except the space through which a wheel works, the rim of which is in breadth suitable for the axle journal, and has formed across it, all round, concave recesses which contain the lubricant. The rim of the wheel, instead of being rigidly connected to the boss or centre by arms, has an elastic connection by means of four spiral springs enclosed in tubular cases interposed between the rim and the boss (which has four flat faces), upon which the feet of the tubular cases enclosing the springs slide freely, the result being that when the axle of a running tub comes into contact with one of the concave recesses in the rim, the journal receives a charge of lubricant, and, besides revolving the rim, pushes it in the direction in which it is running, the springs yielding, and the feet of the enclosing cases sliding on the flat faces of the boss, the rim being pushed temporarily into a position eccentric to the centre of its axle till the tub axle has passed over, when it returns to its normal position, to be acted upon in the same manner by the next following axle, which receives a fresh charge of lubricant. Thus the rim will suit itself to tubs having wheels of different diameters to the extent of several inches.

"The greasing wheel on one side of the line is entirely unconnected with the wheel on the other side; each acts freely to administer a charge of grease to the journal on its own side of the line of tubs.

"Where the tub wheels revolve on the axles this greaser is useless, but is very efficient for tubs to which the wheels are fixed to the axle and the whole revolve."

The Hardy Patent Pick Company, Limited, Sheffield, supply a self-lubricating pedestal for colliery tubs which is designed also to protect the axles of the tubs from dust. This is of much importance, especially in dusty mines, where ordinary bearings get full of coal dust which renders useless the newly applied lubricant. The construction of the bearing is very simple. The top portion bearing the load is similar to the old form of pedestal, but underneath a steel dish is fitted, which keeps the axle free from dirt and at the same time prevents it from leaving its bearing. The steel dish is stamped out of one sheet of metal, and is shaped to hold a quantity of felt or wool, which bears against the axle on the under side, and being soaked with the lubricant, keeps the journal well oiled. When first charged about 5 oz. of oil is required in the operation, but when necessary to replenish, a charge of 2 oz. is sufficient. It is said that the tubs will work two or three months without further attention, after once oiling, and the journals remain clean and well-oiled throughout that time. A tub with outside bearings may have the lubricant applied to the journals at any time as the tub rests on its wheels, but with inside bearings it is necessary to turn the tub by means of a lifter and then the oil may be poured through a hole which leads into a large hollow space in the top of the bearing plate. On returning the tub to its usual position on its wheels the oil runs down into the wool or felt.

CHAPTER XIX.

MISCELLANEOUS QUESTIONS AND ANSWERS.

Denudation-Meeting of Cages in a Shaft with Flat Winding Ropes-Outstroke and Wayleave -Pressure of Water against a Barrier-Applying the Result Obtained from BoringDischarge of Water from a Bore-hole-Pressure per Square Inch Produced by an AirCompressing Engine-Description of Anthracite, Semi-Bituminous, Bituminous, Cannel, and Lignite Coals-Dynamometer-Elements of Rocks-Relative Hardness of “Rock Masses"-Beds and Veins of Minerals-Difference of Vein, True Fissure Vein, and Lode.

Question 152.-What is meant by denudation?

By denudation is meant the wearing and carrying away of the solid materials of the land by the agency of water. Rivers so carry away portions of the land through which they flow; the tidal currents of the ocean lay bare the rocky materials of its shores by wearing away their superficial deposits and by sweeping away the solid masses. The effect of the tidal current of the German Ocean is to sweep away large masses along the eastern coast of England and to make a gradual inroad in the land annually. The soil and other matters transported by the action of rivers from inland situations are brought to the sea and borne by the ocean currents to a much greater distance from their original site. It has been computed that the Hoang-Ho, one of the largest and most rapid rivers in China, brought down in a single hour two million feet of earth. The 1890 Ordnance survey in Yorkshire proved an erosion of the coast there. A portion of the coast line between Great Colden and Dimlington has been almost uniformly encroached upon, since 1852, by the sea for about 215 feet, or at the rate of 5 feet 10 inches per annum.

Question 153.-At what point in the shaft would the cages "meet" if flat ropes, of an inch thick, are used for winding; the drums are 10 feet in diameter at the lift, and the depth of the pit 612 feet?

In winding with a flat-rope drum each coil of rope is wrapped upon the preceding coil every revolution that is made, and as the cage is drawn up from the pit bottom the diameter of the drum constantly increases and attains its maximum diameter when the cage reaches the surface. Again, as the cage is being lowered, the effective diameter of the drum constantly decreases and reaches its minimum when the cage has arrived at the shaft bottom. The increase in the diameter of the drum, the first revolution will be equal to the rope's thickness (because the rope in bending on the drum is lengthened on the convex side and shortened on the concave side equally, the centre of the rope retaining its former length), and for each successive revolution after the first the increase of the drum's effective diameter will amount to twice the thickness of the rope.

Then the diameter of the drum at the first and succeeding revolutions will be

and so on, and at the nth revolution the diameter will be represented by x + t (2 n − 1). The circumference will therefore be

Since this series of terms are in arithmetical progression, their sum may be obtained in the usual way, and it will be, n"(x + nt) which is equal to the length of rope wound on the drum, and therefore the depth of the pit. Therefore, d = n" (x + nt). Or by denoting the depth of the pit in yards by D, then