550 
mercury, a dangerous irruption may make itself known in 
two seconds. A source of great danger is that state of the 
pit which arises from the gradual bleeding of gas from the 
coal. As one walks in a pit one hears a continual click, 
somewhat like the noise of a cricket. In some pits this may 
arise from the settling down of the shale and cracking of 
the coal, but the experienced ear soon knows the difference. 
Should any obstruction arise to the ventilation, this bleeding 
very gradually raises the atmosphere from zero (the point of 
purity) to the point of explosion; or it may be that a 
gradual fall of the barometer admits of the oozing out of 
gas either from a goaf or from the mass of coal, and this, 
although very minute, may be to such an extent as to render 
explosive the whole air of a pit, if the ventilation be not 
very good. There are parts of the pit where gas may so 
accumulate in half an hour ; others where it may be two 
hours, and again others a whole day, in rising to a dangerous 
mixture. To meet such cases, Mr. Ansell used, in the first 
instance, an india-rubber balloon; but this, although per- 
fectly good for its purpose, was too fragile. Therefore, he 
has abandoned that plan, and he now uses white Sicilian 
marble, and, by regulating the thickness, avoids the evil of 
effusion to such an extent that he can, and does, in the pit, 
practically show these gradual accumulations. Mr. Ansell 
has found that by replacing the Wedgewood ware tile (in 
fig. 1) with marble of a quarter of an inch thick, he can 
show the presence of an explosive mixture of fire-damp and 
air which has been half an hour in forming, or by using 
marble half an inch thick, he shows if it has been two hours 
rising from zero to the explosive point. In all such cases, 
notice, if it be so desired, may be given before the mixture 
becomes explosive, the warning being continued for a con- 
siderable time, even if the atmosphere does not become more 
foul. It must, however, be understood that the very circum- 
