The depth of the winning having been determined upon, the manner in which a winning is made by a day level has been described. We have now to state the manner in which a winning is made by machinery.
The depth at which the coal is to be won regulates the power of the engine to be applied, having in view the probable quantity of water which may found, which last regulates the width of the working barrel of the pumps. In a district of country where coals are working by means of numerous winnings, the quantity of water which is likely to be found in a new winning may be estimated from what is found in the other works adjoining; and it has been found from experience, that even in opening up collieries in new districts of coal fields, the water found in sinking is generally such as can be drawn by pumps of from ten to fifteen inches diameter, excepting in eases where the strata arc con nected with rivers, sand-beds full of water, or marshy ground.
In cases where feeders of water proceed from rivers or sand beds, they can be prevented from descending the pit, and therefore the growth arising from these sources needs not be taken into the calculation of the water to be drawn ; and it is found in sinking pi's, that although the growth, which cannot by any means be pre vented from descending into the mine, may he very great, and even so great as to exceed the power of the engine for some time, yet, as the extra How of water fre quently proceeds from the drainage of fissures, these gradually abate to a moderate quantity. An engine having eight or ten hours work drawing water each 24 hours, is reckoned moderate and comfortable for a new colliery. In the course of years, as the workings ad vance, many fissures are cut, and as the coal is excavat ed towards the crop, and approaches the alluvial cover, a constant increase of water is found, so that a colliery which has been long at work frequently becomes heavily loaded with water, and the machinery is required to go night and day. When a colliery is thus loaded with the ordinary daily growth of water, its situation is rendered dangerous and uncomfortable, to obviate which, the potter of the engine is either increased, or additional engines erected. In practice, working barrels are sel dom used of gi eater diameter than 18 inches, and in place of having pumps so large, many ol the mining engineers prefer having two engines with less powers, and mode rate sized working barrels, on account of the unwieldy nature of such heavy machinery, and the great extra tear and wear attending them.
The extent of the winning to be made, as to depth, and the power of machinery to be employed, having been determined upon, the mode of sinking engine pits has next to be described.
Fir8t, When the depth is moderate, and the process comparatively very simple.
Second, When the depth is great, and the process la borious and intricate.
In any winning, the figure of the engine pit is a pri mary consideration. In winnings of a moderate depth many forms are used, as circular, oval, square, octago nal, oblong rectangular, and long-elliptical.
In pits of inconsiderable depth, and where the alluvial cover is of a firm and dry consistency, any shape thought the most convenient may be used ; but in all deep pits, no shape but the circular ought to be used. Indeed, when water requires to be stopped by tubbing or crib bing, no other shape will do, the circular shape being the only one which presents a uniform resistance in all points to the uniform and great pressure of water—even the elliptical shape, where there is but little difference between the two diameters, is not suitable ; it has been tried, and has always given way when exposed to pres sure of any considerable degree. Besides, the circular shape renders the pit stronger in the shaft walls, and is likely to be less injured than any other shape, in the event of the shaft being shaken by a sit or crush arising from the failure of the pillars left in working the coal. As to the sizes of engine pits, the smallest should be ten feet diameter, to admit the pumps to be placed in the lesser segment, and the coals to be drawn at the larger one, as in Plate CCCXC Fig. 9. which is termed a dou ble pit ; but if much wut k is to be done in drawing coals, and particularly if the coals are large, it would he an economy to have the pit wider than ten feet. When a pit is to he divided into three divisions, one for the en gine pumps, and two for drawing coals, as in Fig. 10, which is termed a triple ph, it would require to be twelve feet di. meter. If it is to be divided into four di visions, and made a quadrant pit, as in Fig. II. with one division for the pumps, and three divisions for ventilation and coals drawing, it would require to be fifteen fret diameter. These sizes of pits are regulated by local cir cumstances, and by the output of coals proposed to be raised.