1. _Irregular layers of sandstone_, appearing in the middle of the
coal-seam, and gradually increasing in thickness till they separate the
coal into two distinct seams, too thin to continue workable.
[Illustration: 817 818]
2. _Nips_, occasioned by the gradual approximation of the roof and
pavement, till not a vestige of coal is left between them; the softer
shale disappearing also at the same time. _Figs._ 817. and 818.
represent this accident, which is fortunately rare; the first being a
vertical, and the second a horizontal view.
3. _Shaken coal._ It resembles the rubbish of an old waste, being a
confused heap of coal-dust, mixed with small pieces of cubical coal, so
soft that it can frequently be dug with the spade. This shattering is
analogous to that observed occasionally in the flint nodules of the
chalk formation; and seems like the effect of some electric tremor of
the strata.
In searching for coal in any country, its concomitant rocks ought to be
looked for, especially the carboniferous or mountain limestone, known by
its organic fossils; (see Ure’s Geology, p. 175, and corresponding plate
of fossils;) likewise the outcrop of the millstone grit, and the newer
red sandstone, among some rifts or façades of which, seams of coal may
be discerned. But no assurance of coal can be had without boring or
pitting.
Skill in boring judiciously for coal, distinguishes the genuine miner
from the empirical adventurer, who, ignorant of the general structure of
coal-basins, expends labour, time, and money at random, and usually to
no purpose; missing the proper coal-field, and leading his employer to
sink a shaft where no productive seams can be had. A skilful viewer,
therefore, should always direct the boring operations, especially in an
unexplored country.
The boring rods should be made of the best and most tenacious Swedish
iron; in area, about an inch and a quarter square. Each rod is usually 3
feet long, terminating in a male screw at one end, and a female screw at
the other. The boring chisels are commonly 18 inches long, and from 2
inches and a half to 3 inches and a quarter at their cutting edge, which
must be tipped with good steel. The chisel is screwed to an intermediate
18-inch rod, called the double box-rod, forming together a rod 3 feet
long. There are, moreover, three short rods, a foot, 18 inches, and 2
feet long each, which may be screwed, as occasion requires, to the
brace-head, to make the height above the mouth of the bore convenient
for the hands of the men in working the rods. Hence the series of rods
becomes a scale of measurement for noting the depth of the bore, and
keeping a journal of the strata that are perforated. The brace-head rod,
also 18 inches long, has two large eyes or rings at its top, set at
right angles to each other, through which arms of wood are fixed for the
men to lift and turn the rods by, in the boring process.
When the bore is intended to penetrate but a few fathoms, the whole work
may be performed directly by the hands; but when the bore is to be of
considerable depth, a lofty triangle of wood is set above the bore-hole,
with a pulley depending at its summit angle, for conducting the rope to
the barrel of a windlass or wheel and axle, secured to the ground with
heavy stones. The loose end of the rope is connected to the rods by an
oval iron ring, called a runner; and by this mechanism they may be
raised and let fall in the boring; or the same effect may be more simply
produced by substituting for the wheel and axle, a number of ropes
attached to the rod-rope, each of which may be pulled by a man, as in
raising the ram of the pile-engine.
In the Newcastle coal district there are professional master-borers, who
undertake to search for coal, and furnish an accurate register of the
strata perforated. The average price of boring in England or Scotland,
where no uncommon difficulties occur, is six shillings for each of the
first five fathoms, twice 6 shillings for each of the second five
fathoms, thrice 6 shillings for each of the third five fathoms, and so
on; hence the series will be--
1st five fathoms 6_s._ each _£_1 10
2nd five fathoms 12_s._ -- 3 0
3rd five fathoms 18_s._ -- 4 10
4th five fathoms 24_s._ -- 6 0
-- ------ --------
20 fathoms of bore _£_15 0
Thus the price increases equably with the depth and labour of the bore,
and the undertaker usually upholds his rods. There are peculiar cases,
however, in which the expense greatly exceeds the above rate.