20 
POPULAK SCIENCE KEYIEW. 
been secured, n is the downcast, y the upcast shaft, and the 
bed of coal rises from n to v. The first thing done on begin- 
ning to work the mine is to drive out roads, n a, n b, as 
nearly level as possible, to the right and left of d up to the 
boundaries of the royalty ; the roads a a', d v, b b', are then 
driven at right angles to a b, up to the boundary on the rise 
side : the boundary on the dip side is a b. The extraction of 
coal now begins, and the mineral is sliced off in long faces 
parallel to a' b', starting at the rise boundary, and working 
backwards to the dip. In this way the “goaf” is always 
behind and to the rise of the workings, and is traversed only by 
the air course ev, which may be bricked and completely 
isolated from the fiery district on either side. The intake air 
is split into two courses, which travel along n a c, n b F, sweep 
the faces c e, f e, and pass along e v to the upcast. The coal 
as it is extracted is drawn along the levels c E, F E, and run 
down the incline e n to the drawing shaft n. 
Very simple indeed this looks on paper, but unluckily in 
practice innumerable obstacles come in the way, when we 
attempt to carry out such a plan. First natural obstacles 
occur. The rise of the coal is not constantly in the same 
direction, and the seam is often traversed by lines of fracture, 
known as “ faults ” or “ troubles,” by which the bed has been 
broken across, and the part on one side raised or depressed 
relatively to the part on the other side. Commercial con- 
siderations give rise to still more serious difficulties. The sub- 
division of property often makes it difficult to obtain a tract of 
coal anything like as symmetrical as that in the sketch. The 
time taken in driving the preliminary roads or “ straight 
work ” would in a large colliery be very considerable ; and 
adventurers do not like to see their money lying idle so long, 
and are tempted to obtain quicker returns by beginning to 
raise coal as near to the shaft as is safe. The position of the 
drawing shaft is mainly determined by the consideration that 
it must be as near as possible to the canal or railway that is to 
carry the coal to market, and it may well happen that this is 
not on the dip of the mine. Unless the mine be very large 
indeed, it will be impossible to obtain from a single face an 
output large enough to make the undertaking pay. These 
and other obstacles, too numerous to mention here, prevent us 
in practice from ever obtaining all the advantages possessed by 
the plan shown in Fig. 1, and corresponding compromises 
have to be made ; but the general principles there illustrated 
are such as guide the engineer in the laying out of a colliery. 
The precautions already described would render a mine, in 
which gas exuded uniformly and slowly from the coal, abso- 
lutely safe ; but they do not alone suffice where the outbursts 
