551 
stances which admit of the air becoming " fouled " to the 
extent of ten per cent, also admit of its going on till fifty or 
more per cent, has been reached, always remembering that 
these instruments are to be fixed high up in the roadway 
or goaf, and that the gas accumulates there notwithstand- 
ing the law of diffusion. It is no uncommon thing to 
find thirty per cent, of gas next the roof, at six inches 
lower twenty per cent., and at fifteen inches no gas at all. 
The cause of this will be discussed on some other occasion. 
It is proposed to fix the instruments side by side, one 
for sudden and the other for slow accumulations, in pigeon- 
holes, being clear all through, so that the gas can surround 
the instruments or sweep over them ; for the gas as it occurs 
in the pit is very curious in its habits, and, from causes too 
minute to enumerate here, it ' goes away ' from a spot with 
very little ' disturbance.' The pigeon-holes being formed 
in iron posts would protect the instruments from falling roof, 
&c, while grooves may be cast in the sides of these posts for 
the telegraphic wires. It has been objected by some that 
these instruments would cause greater destruction of life 
than now obtains ; but these persons forget that they are not 
intended to displace other means of safeguard. They are 
simply proposed as additional means of knowledge. For the 
indication of carbonic acid Mr. Ansell makes a necessary 
alteration, which will be seen in fig. 2. This hardly needs 
