COAL MINING. 365 
admit the passage of columns of cool air between the farnace and pillars 
of coal for the purpose of preventing the pillars taking fire, but the 
chambers are found in practice to add to the amount of current. 
Where the air-ways of a mine are of, say 30 feet of sectional area, 
a furnace 7 feet wide and 3% feet high above the bars will, I think, 
approach systematic perfection. The furnace has a limit to its power, 
and when that is reached we pile on coals in vain. In building a fur- 
nace, it adds to its efficiency to slant it upward inside the bars, say 1 
foot in 6, until the upcast shaft is reached. 
In the mines of this State the quantity of air moved by a properly 
constructed furnace ranges from 2,000 to 6,000 cubic feet per minute 
for every foot of breadth of fire. The depth of the ventilating shaft, 
its freedom from water, the sizes of the air-courses of the mine, the 
temperature of the outside atmosphere, all combine in determining the 
quantity of air which can be moved through a mine by furnace ventila- 
tion. In winter, as stated in the opening paragraph of this paper, the 
natural forces aid the ventilation, while in summer the natural forces 
oppose the furnace, like a steam-boat going up stream. In deep mines, 
like those in England, the natural current is in the direction of the up- 
east all the year round, because the mine air of deep mines is always 
rarer than the atmosphere on the surface ; but, while in summer there 
is no opposing force to overcome, there is little assistance given, the 
temperature of the mine and surface air being so nearly equal in weight. 
In winter the natural forces and the furnace are in proportion to the differ- 
ence of temperature of the mine and surface air. The practical power 
of the furnace is in proportion to the depth of the shaft, the power 
being as the ratio of the depth; hence, a shaft 400 feet deep will, with 
the same furnace, all other things being equal, move twice as much 
air as a shaft 100 feet deep. This practical fact is not as well under- 
stood as it should be, the common impression being that shallow mines 
move more air than deep ones, with the same ventilative power. Until 
within a few years ago, it was a rare thing to see a roomy, well-con- 
structed furnace in a coal mine in this State, owing to the mistaken 
view of the influence of heated air in shafts. 7 
Fan-ventilation, on the ‘other hand, is more effective in shallow 
than deep mines, but fan-ventilation has only recently been applied in 
this State, and is not making as rapid headway as could be wished, 
mainly from the fact that the first cost of the fan is considerably greater 
than that of the furnace, and in drift mines it is as costly at all times, 
