362 GEOLOGY OF OHIO. 
labyrinthian passages of the mine, it becomes more and more vitiated and 
unfit for breathing from the loss of oxygen, which is replaced by the 
noxious and poisonous gases met on the way. When we consider the 
numerous complaints which reach the public ear, over the condi- 
tion of badly ventilated public buildings and workshops, and remem- 
ber the numerous treatises which have been written on the best methods. 
of improving the ventilation of such buildings, we are forcibly reminded 
of those dark subterranean workshops, amidst which the causes which 
tend to vitiate the atmosphere are multiplied a hundred fold, and where: 
not even a ray of sunlight can ever come. 
In laying out the workings of mines, two general systems are 
adopted, with the view as well to provide the means for circulating 
currents of air through the workings, as for mining away the coal bed, 
namely: the long-wall system and the pillar and room system. By the 
former method all the coal is withdrawn as the works advance pro- 
gressively forward, the overlying strata being allowed to fall down and 3 
close in behind the miners, who maintain traveling ways by cutting up 
the floor or blasting down the roof. In the latter system columns of 
coal are left in the mine as the workings advance, for the support of the 
superincumbent strata, these columns being attacked afterwards ; some- 
times in a series of rooms as the workings advance, but more generally 
after all the rooms have been finished up to the boundary line of the 
mining plant. Long-wall mining, although it can be employed to more 
advantage in many seams of coal than pillar and room practice, has not 
yet obtained a foothold in Ohio mines, all our coal being won by the 
pillar and room system. 
Pillar and room working, as its name indicates, consists in forming 
pillars and rooms, alternately; the proportion of coal mined away to 
that left standing being governed by circumstances and conditions. 
Rooms are made wide and pillars narrow when the roof is hard and 
firm, and the thickness of the overlying strata is not great; when the 
roof is tender and the superincumbent strata heavy, narrow rooms and 
strong pillars are required. : 
In opening a mine on the pillar and room system, gang-ways, 
entries, headings or galleries, as they are variously called, are first run 
forward on the face and end-slips of the coal bed. These entries, which 
in all well-regulated mines are made double, constitute the main avenues 
of the mine; they are usually driven much narrower than the rooms or 
