1. The lowest coal of the winning should be worked in such a manner as not to injure the working or the value of the upper coals of the field; but if this cannot be done, the upper coals should be worked in the first place.
2. The coals must be examined as to texture, hardness, softness, the number and openness of the backs and cutters.
3. The nature of the pavement of the coal seam, particularly as to hardness and softness; and if soft, to what depth it may be so.
4. The nature of the roof of the coal-seam, whether compact, firm, and strong; or weak and liable to fail; as also the nature of the superincumbent strata.
5. The nature of the alluvial cover of the ground, as to water, quicksands, &c.
6. The situation of rivers, lakes, or marshes, particularly if any be near the outcrop of the coal strata.
7. The situation of towns, villages, and mansion-houses, upon a coal-field; as to the chance of their being injured by any particular mode of mining the coal.
Mr. Bald gives the following general rules for determining the best mode of working coal:—
“1. If the coal, pavement, and roof are of ordinary hardness, the pillars and rooms may be proportioned to each other, corresponding to the depth of the superincumbent strata, providing all the coal proposed to be wrought is taken away by the first working, as in the first system; but if the pillars are to be winged afterwards, they must be left of an extra strength, as in the second system.
“2. If the pavement is soft, and the coal and roof strong, pillars of an extra size must be left, to prevent the pillars sinking into the pavement, and producing a creep.