18 
POPULAR SCIENCE REVIEW. 
back the air current ; or in some cases simply by hanging heavy 
cloths across an opening through which it is wished to prevent 
the passage of the air. Sometimes, when the ventilating cur- 
rent has to cross one of the main roads which it would be 
inconvenient to interrupt by doors, the air is carried across in a 
distinct passage over the roof of the gallery. 
Thus there is constantly going on a process which does for 
the mine exactly what the functions of breathing and the cir- 
culation of the blood do for the human body ; and we must 
next see by what means the circulation is kept going in the 
case of the mine. 
The methods usually employed are twofold : sometimes a 
large fan, turned by machinery, is placed at the top of the up- 
cast shaft, to create an upward draught ; sometimes a furnace 
is kept burning in that shaft, which rarifies the air, causes it 
to rise, and sucks a corresponding quantity to fill its place down 
the downcast. 
The disadvantage of a furnace is that the returns may be so 
charged with fire-damp as to explode on coming in contact 
with the flame. To obviate this the foul air is carried through 
a gallery, known as a “ dumb drift,” which opens ii^to the up- 
cast some way above the furnace : even this precaution, how- 
ever, is thought to have been ineffectual in some very fiery 
mines, and good authorities have given it as their opinion that 
there are cases where no precaution can render a furnace safe, 
and that a fan ought then to be employed. One great draw- 
back to the use of a fan is, that the instant any accident 
happens to it, or the machinery which turns it, ventilation is at 
once stopped ; whereas with a furnace, especially if it be placed 
at the bottom of the upcast shaft, and that shaft be deep, a 
long time will elapse before the heated column of air can cool 
down to the same temperature as the rest of the mine, and ven- 
tilation, to a certain extent, will go on after the furnace has 
gone out, should either accident or carelessness cause that to 
happen. Hence, when fans are used, it is very desirable to 
have at least two, so that if one be disabled the other may 
immediately take its place. 
One or two other points connected with ventilation call for 
notice. Beds of coal very rarely lie perfectly flat ; in most 
cases they are inclined, or “ dip ” at various angles to the 
horizon. Now, the small specific gravity of fire-damp tends to 
make it flow of itself towards the highest, or “ rise ” parts of 
the mine, and the tendency should be taken advantage of to 
facilitate the escape of the gas. Hence it is desirable to put 
the upcast shaft as far to the rise as can be done. Further we 
have noticed the tendency of gas to accumulate in abandoned 
workings or “ goafs,” from which falls of the roof are liable to 
