COAL MINING. | 363 
chambers, so as to make them extra safe, as well as to add strength to 
the pillars. The rooms are invariably started off the butt entries of 
the mine. In both entries and rooms, break-throughs require to be 
made from one working-place to another at stated intervals for the 
passage of the ventilating currents of air. 
In all of the mines in this State in which improved mining systems 
are understood, no working-place is driven forward more than 40 yards 
ahead of the circulating currents until a break-through is cut in the 
pillar from one working-place to another. All break-throughs, except 
those last made near the working faces of the mine, are built up and 
rendered air-tight by brattice, trap-doors, or otherwise, in order to force 
the air-currents forward where the people are employed, for the ten- 
dency of the current is to follow the easiest route to the upcast. 
In mines in which no fire-damp is given off, fully 100 cubic feet of 
air per minute should be circulated per man; in mines which make fire- 
damp a much greater quantity is required, particularly if the fire-damp is 
emitted copiously. But this current must be made to sweep through 
the interior of the mine, where the men are employed, or it will do 
little or no good. ‘There may be ten times the amount of air required 
for the sanitary condition of a mine entering by the intake, and dis- 
charged by the upcast, and yet the working-places in the interior be in 
a very defective condition. Under every system of ventilation there is 
a loss of air by leakage. 
When two separate openings of different depths are made into a 
mine, a current of air is set in motion by the natural pressure of the 
atmosphere. In winter the lower opening will be the downcast, and in 
summer it will be the upcast, because during winter the atmosphere 
outside is denser, and consequently heavier than the air of the mine, 
while in summer the reverse is the case. During those seasons of the 
year in which the mine atmosphere and the air outside approximate 
each other in density, there will be no motion, or it will be so slight as 
to be of little service. 
As underground excavations become more extensive, the natural 
forces, even during seasons most favorable to their operation, become 
wholly inadequate as a ventilating power, owing to the resistance which 
the top, bottom, and sides ot the air-way offer to the moving current of 
air, and artificial ventilation has to be applied to produce the circulation 
required to sweep away the gases and render them harmless. Furnaces 
