C.—3

30

The walls are built with studs and a lining, C. The latter maybe of 2 in. tongued-and-grooved timber, or of two thicknesses of flooring-boards laid diagonally across each other. On the inside of th"'s lining the two circular cribs A, A, are to be accurately fixed and secured with bolts. The segments of the cribs should be scarf-jointed, and the joints plated above and below. The contracting cheek B and expanding cheeks b are to be screwed to the segments A and the wall-lining C, and must be very cose fitting. As the fan-blades are to revolve as closely as possible to the contracting cheeks B without touching them, the timber for these cheeks must be dressed smooth and true. All timber should be of well-seasoned heart-wood (kauri, rimu, or matai), and receive three or four coats of tar before being put together. After erection all the inside wood- and ironwork should have at least three coats of thick tar, applied hot. An extra coat or two of tar will be an advantage, and enough should be applied to the inside to form a good strong glaze. Too much care cannot be observed in doing this before the fan is set to work, as subsequent coats cannot be applied owing to the moisture contained in the air exhausted from the mine making everything slimy and wet. Where the main shaft passes through the lining C, the latter should be cut away for a few inches round the shaft, and the space so formed covered with a light iron plate made in two halves (each half having a semicircular opening cut out, so that when placed in position a hole very slightly larger in diameter than the diameter of the shaft is formed for the latter to work through), and bolted on to the linings. The object of this is to isolate the woodwork from the bearings, and so minimise the possibility of such a contingency as the firing of the bearings igniting the woodwork. The quantity of air which a fan of given size will circulate through the workings of mines cannot be stated with accuracy owing to the very different conditions of various mines. Much depends on the length, area, and state of the airways, and the position the fan will occupy as regards the conditions affecting natural ventilation. To obtain the best results the fan should be erected so as to exhaust from the shaft which nature selects as the upcast, and, if possible, all steam-pipes from the surface to pumping or hauling machinery below ground should be fixed in the same shaft. The circumferential velocity of fans—i.e., at the tips of the blades—varies in practice from 4,000 ft. to over 8,000 ft. per minute. The mean of these, or, say, a circumferential velocity of 6,000 ft. per minute, will be found a fair speed for the simple fan now illustrated and described, the diameter of which may be from 5 ft. to 13 ft. 6 in. or upwards. The larger size named will be found ample for the requirements of the most extensive mine now at work in New Zealand if placed under anything like favourable conditions. For driving, a steam-engine would naturally be used at collieries, and also at some quartz-mines where water-power is not available, and the idea that any old engine will do should not be entertained. An engine for driving ventilating machinery should be first class, and not liable to be stopped frequently for repairs, and of a type which will work economically as regards consumption of steam. Where water-power is available, as at many quartz-mines, a Pelton wheel will be found a suitable motor for driving.

IMPROVEMENTS IN PUMPING AND AIR-COMPRESSING MACHINERY. (With Special Reference to the Riedler System of Controlled Valves.) During the last few years the method of pumping mine-water has been gradually changing. At new mines especially, pumps placed underground, and forcing water from the bottom to the surface, have been adopted in preference to the old style of pumping-engine erected above ground, whilst at some older mines the latter style has been either superseded or supplemented by pumps placed at the bottom of the shaft. The initial cost of large single-acting draw-lift, and plunger pumps, with their heavy rods and pipes, as well as massive engines, beams, gearing, foundations and buildings, is very considerable when compared with that of a modern high-class plant of equal capacity placed at the shaft-bottom. So far back as 1876 a plant of this kind was erected at a large mine in England at which I was then engaged. The pump forced water direct to the surface, a distance of about 1,000 ft. vertical, and practically demonstrated the possibilities of the system. Since then similar methods have been adopted for much greater depths. Power can be, and is, transmitted from the surface to underground pumps by steam, compressed air, electricity, and endless wire ropes ; also to a limited extent by a system of hydraulics. At most collieries and some metalliferous mines steam will continue to be used direct, but there is no reason why mine pumping-engines should be so extravagant in steam-consumption as they often are. It is quite possible to work them on the same economical lines as marine engines, their principal drawback, as compared with the latter, being the distance away from the boilers (which naturally will be erected on the surface); but when steam is generated at a fairly high pressure, and conveyed to the pumps in well-laid pipes of ample area, properly fitted with expansion-joints and thoroughly covered with a good composition of non-conducting material by which the heat will be retained and condensation minimised, the loss of pressure is very small, and with suitably designed engines a reasonably high degree of efficiency may be maintained. In some instances pumps are driven at a slow rate of speed by quick-running engines, the connection between engines and pumps being made by spur-gearing ; but in others the engines are, by the continuation of the piston-rods, coupled direct to the pumps. In the latter case the piston-speed of the engines is no more than that at which the pumps may be run with safety, and is consequently too low for real economy. Modern practice in engine-building is based on the lines of greater efficiency of steam with a moderately high rate of piston-speed. Engines now built for driving mill machinery, electrical plants, steamships, Ac, are frequently designed for piston-speeds of from 500 ft. to over 1,000 ft. per minute. With pumps as ordinarily constructed anything