COMMON COAL. 153 
of coal, which lie at a considerable distance above each 
other, and which have a communication by pits that are 
sunk between them. These strata are not always regu- 
larly continued in the same plane. The miners occa- 
sionally meet with veins of hard rock, which interrupt 
their further progress, and, at such places, the earth, on 
one side of the vein, appears to have sunk down, while 
that on the opposite side has its ancient situation. 
These breaks the miners call dykes (4<). When they 
come to one of them, their first care is to discover whe- 
ther the coal, in the part adjoining, be higher or lower 
than that in which they have been working ; or, to use 
their own terms, whether it be cast down or cast up. 
For this purpose they examine attentively the mineral 
strata on the opposite side, to see how far they corre- 
spond with those which they have already passed 
through. If the coal be cast down, they sink a pit to 
it : but if it be cast up, the discovery of it is often at- 
tended with great labour and expense. 
In general the entrance to coal mines is by perpen- 
dicular shafts, and the coals and workmen are drawn up 
by machinery. As the mines frequently extend to 
great distances, horizontally, beneath the surface of the 
earth, peculiar care is necessary to keep them conti- 
nually ventilated with currents of fresh air, for the pur- 
pose, not only of affording to the workmen a constant 
supply of that vital fluid, but also to expel from the 
mines certain noxious exhalations which are sometimes 
produced in them. 
One of these, denominated fire damp, is occasioned 
by the generation of hydrogen gas, or inflammable air 
(45). This gas, when mixed with the common air of 
the atmosphere, explodes, with great violence, on the 
approach of a lighted candle, or any other flame ; and 
has, at different times, occasioned the loss of many valu- 
able lives. It is a singular circumstance, that although 
it is immediately set on lire by a flame, yet it cannot be 
kindled by red hot iron, nor by sparks produced 
from the collision of flint and steel. Hence a machine 
H5 
