116 
ditions such as these. No amount of precaution can equal 
all the emergencies which may arise, so long as the old 
system is made to answer the purposes of vastly extended 
areas. The whole proceeding as regards ventilation is per- 
plexing and difficult, and this from the simple circumstance 
that the theory adopted is in direct antagonism to that 
which the gas obeys as a law, and in which obedience it 
perseveres with an insiduous activity which no human 
observation or skill can at all times detect or command. 
It is from the peculiar conditions to which reference has 
now been made, that a coal mine, after being worked for a 
time, becomes a vast reservoir in which every cavity becomes 
charged with gas or air, in greater or less degree of purity ; 
and before this reservoir can be penetrated with safety, the 
air must be extracted, and a fresh supply admitted, in which 
human respiration can be carried on. The circumstances are 
analogous to a reservoir on the surface, filled with impure 
water, which it is desirable to replace with liquid of a 
more satisfactory quality. The process of emptying may 
be performed either by pumping the water over the embank- 
ment, or by allowing it to escape by a perforation made 
through the base of it. In the latter case advantage is 
taken of the gravitating force impressed by nature on the 
element itself, and with a freedom and facility commanding 
our admiration and delight, the operation is performed 
without labour or expense to ourselves. In the former case 
the work is equally practicable. Pumps may be resorted to, 
and by their long, tedious exercise, the complete exhaustion 
may be consummated. Precisely the same holds good with 
the subterranean reservoir of gas. Expelled by means of 
an up-cast, placed upon the dip of the mine where gas is 
generated, the act of exhaustion becomes strictly analogous 
to the pumping operation on the surface. In both instances 
the element is made to obey an impulse to which it has a 
