306 GEOLOGY OF OHIO 
sometimes driven forward to the boundary of the mining property 
before the walls are opened out, and in excavating the coal, the miners 
retreat instead of advancing with their work; sometimes, also, in pillar 
and room practice the same plan is adopted, the main galleries of the 
mine being driven to the boundary before any of the rooms are 
opened out. ) 
Mr. Robert Bald has given the following general rules for deter- 
mining the best method of working coal by pillar and room practice: 
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 ; but if the pillars are 
to be winged, or partially worked afterwards, they must be left of an 
extra strength. 
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. 
3. If the coal is very soft, or has numerous open backs and cutters, 
the pillars must be left of an extra size, otherwise the pressure of the 
superincumbent strata will make the pillars fly or break off at the backs 
and cutters, the results of which would be a total destruction of the 
pillars, termed a crush or sit, in which the roof sinks to the pavement, 
and closes up the work. 
Regarding the long-wall system, the late Matthias Dunn gives the 
following conditions under which it may be adopted to advantage : 
1. If the coal be thin, hard, and capable of bearing pressure, or 
_the top and bottom be soft, requiring a considerable quantity of cutting 
for the necessary tram height. 
2.° If band or rubbish be mixed with the coal, requiring to 
be stowed underground, so as to furnish a cheap and plentiful supply of 
debris for filling. 
3. If the distance from the shaft to the boundary line be limited. 
4. If the roof be free from water, and the workings clear ot 
buildings, rivers, ete. 
5. If the roof contain ironstone, to be worked with the coal, 
thereby producing much refuse, or if it be so soft and brittle that it 
will not stand for ordinary pressure. 
Under these, and many other circumstances, the long-wall system 
