COAL MINING. 307 
may be recommended. There may, however, be objections, rendering 
such working impracticable, viz. : 
1. If the workings produce a considerable quantity of inflammable 
gas, either from the seam itself or from some superior seam. 
2. If the roof contains water, the letting down of which would 
spoil the tram-ways or overpower the engine and pumps. 
3. If the coal be so near the surface that the long-wall workings 
would have the effect of damaging buildings. 
4. If the cuttings of the roof or floor for height are so soft or 
friable that they would not be sufficient to support the roads, in which 
case the expense of setting additional props or obtaining other material 
may exceed the value of the coal, or its cost by another system. 
5. If the seam be deep, and from its thinness suitable for long- 
wall, yet the small quantity ordinarily produceable from one establish- 
ment may render its working unprofitable ; or, in other words, the main- 
tenance of expensive roads, or the number of pits required, may not be 
repaid by the working thereof. 
In the first system-—working hy pillars and rooms—the mines are 
laid off by driving water levels or galleries along the line of strike of 
the coal from opposite sides of the pit bottom. On the dip-side of the 
gallery a small quantity of water follows the workman, and serves as a 
guide in advancing the work. The main part of the workings is 
driven to the rise of the coal strata, for in British mines the coal meas- 
ures have generally a well-defined dip and rise. Advantage is taken of 
the face and butt slips, and the rooms are worked on the face of the 
coal whenever it is practicable to do so, as the coal is much easier 
worked on the face than obliquely. Three-fourths of the coal are 
usually taken out, the remaining fourth being left for pillars. A com- 
mon practice is to make the rooms of the same width and size of the 
pillars left; thus, where the rooms and cross-cuts are each fifteen feet 
wide, the pillars are made fifteen feet square; to add strength to the 
pillars, they are frequently formed into diagonal blocks, like the black 
squares of a checker-board. This system is only practicable in shafts 
of comparative shallow depth, and where the coal, roof and floor are 
alike hard, firm and compact. Under other conditions a crush or creep 
of the workings is sure to follow, before the domain sought to be won 
is half wrought over. This plan is now rarely practiced in British 
mining, and will soon disappear altogether. Its advantages consist in 
y 
