Coal prefents feveral varieties, and is
more cr left interefting; fome are fhiftofe,
hard and fulphureous; others are foft, and
quickly confume.
The beft coal is generally of the leaft
fpecific gravity, and of the brighteft black
colour, finely laminated, and on burning
leaves the leaft afhes.
The cannel or candle coal is very com-
paT ; fraCture fplintery; it is lighter than
the other variety, and is fonorous when
{truck; frequently explodes when heated,
and burns with a luminous flame ; its colpur
is jet black; it is capable of a fine polifh j
it feems to contain more carbon and lefs
fulphur; it is fometimes found under and
in connection with the common varieties.
Coal is fometimes in contaCt with afphalt
and indurated bitumen.
Coal mines are of various depths; and
coal often baflets * out to the grafs, The
* See the gloflary at the end.
firatum
ftratum is frequently broken, when the
workmen meet with a fault, mear, or lum,
which is a cavern filled up with clay, or
rubble, difiocating the ftratum of coal. In
fuch cafe the coal is fometimes lifted up, ten
or twenty yards ; or as much thrown down.
See the coal ftratum at B, (fee plate l.) and
the fiflure at F where the coal is thrown
down at D. In fearching for coal, ftreams
of water, after heavy rains, fhould be examined,;
and when it is found, the mo ft eafy
method of working fhould be adopted, that
an article of fuch general ufe may be rendered
as cheap as poffible. It is the grand fource
and root of all our manufactures, and of
the firft national confequence; whence too
much encouragement cannot be given to
fearch for this neceflary article, where it
has not yet been found. It would prove
an inexhauftible fource of wealth in the
Highlands of Scotland: agriculture would
c 3 flourifh/