304 GEOLOGY OF OHIO. 
and after the rooms are all finished, the pillars being in turn 
attacked. There are many modifications of this general system prac- 
ticed in the various regions of the State. 
In laying out the underground workings of mines, a plan of the 
proposed system of working is usually made in advance by a competent 
practical mining engineer. The conditions must be carefully investi- 
gated as to drainage, the nature of the roof and floor, the texture of the 
coal, and the weight of the overlying strata. It is not always practical 
to follow the letter of the plan laid down, owing to the irregularities of 
the floor, and to the presence of horsebacks, but the spirit of the plan 
may be followed to the end. 
The double entry system obtains in all well-regulated mines; it 
consists in carrying forward two galleries in parallel lines on the face of 
the coal; a pillar of coal, 3 to 6 yards in thickness, being left between 
the entries, which is cut through every 30 to 40 yards for air. Asa 
new air-hole is made at the face of the heading, the outer one is closed 
up, and made air-tight by a wooden brattice, or otherwise, so as to force 
the air forward to the working face. Butt entries are opened to right 
and left of the main galleries; they are also made double, and are in 
all respects like the main galleries. Entries are made much narrower 
than rooms; they are generally driven 8 or 9 feet wide, for the purpose 
of having the roof firm and safe. The rooms from which the great bulk 
of the coal is got, are opened in the butt entries. Rooms are started 
at the same width at which the entries are worked, but they are rapidly 
opened out to full width, 7 to 9 yards; they are worked both north and 
south, in lines parallel with the main galleries of the mine. Butt 
entries are 160 to 200 yards apart. 
The pillars or ribs left between rooms are of varying thickness, 
according to the nature and weight of the overlying strata; under a 
firm roof and a light cover, ribs 2 to 3 yards in thickness suffice ; they 
are made thicker in proportion to the weight of the overlying rocks. 
Thin ribs are cut through every few yards, but when they are 4 to 6 yards 
in thickness, break-throughs are less frequently made. These break- 
throughs, like those made between the entries, are cut for the purpose 
of keeping a fresh stream of air as near the face of the workings as may 
be practicable. | 
The pillars of the mine are generally allowed to remain until all 
the rooms are worked out; they are then attacked in the interior of the 
