286 On the Ventilation of Coal-Mines. ([Apriz, 
on the contrary, we can sometimes trace the same individual stratum, 
or number of strata, along the surface, or dead level line, for several 
miles; and, therefore, we may properly call this the longitudinal 
line of bearing. ‘ 
** From these observations it appears that every individual stratum 
in the whole section keeps its station where you see it placed; and 
that it spreads as wide, and stretches as far, as any of those which 
are placed above and below it, which, perhaps, may be for several 
miles every way.” 
** The stratum which is placed immediately above the seam of 
coal is properly called the voof of the coal; and the stratum which 
is placed immediately below a seam of coal is with equal propriety 
called the pavement of the coal. Now these three, that is, the 
stratum of coal, its roof, and its pavement, with the other conco- 
mitant strata lying above and below them, always preserve their 
stations and parallelism; that is, they are all stretched. out, and 
spread one above another, upon the same inclining plane ; and they 
have the same lines of bearing and declivity.” 
To the foregoing may be added, as entering a little more into 
detail, the following passage from the Edinburgh Encyclopedia, 
vol. vi. part 2d, under the article Coal :— 
** Coals, with their various accompanying and parallel strata, are 
found lying in every inclination to the horizon. Some of them are 
vertical, others nearly horizontal, but never absolutely so, to any 
considerable extent. ‘The most common dip or declination is from 
4 m5 to 1 in 20,” 
From the description just quoted, your readers will probably 
be able to acquire a correct notion of the general position of the 
coal strata; and in order to form a tolerably precise idea of a 
coal-mine in this neighbourhood, before it has been opened, they 
have only to suppose a portion of this inclining stratum of coal to be 
** spread out like the face of a country,” to the extent of 1200 
yards square ; that it is six feet thick from the * roof to the pave- 
ment ;” that it dips to the south-east, consequently rises to the 
north-west; and, further, that the south-eastern side of this sup- 
posed square of 1200 yards is 90 fathoms, or 180 yards in perpen- 
dicular depth, while the north-western side is only 80 yards below 
the general surface of the country. This is taking the dip at the 
rate of 1 in 12, nearly the mean of the extremes quoted from the 
Encyclopedia, and which will be found to correspond with the dip 
of ‘several collieries now working. 
Having thus exhibited a field, stratum, or seam of coal, in its 
extent and position in the bowels of the earth, it is necessary to 
state, for the information. of such as are not much conversant in the 
details of coal-mining, that there are many very formidable ob- 
stacles to be encountered and obviated, before it be brought to the 
surface from such a depth. The sinking a shaft, or pit, 180 yards 
in depth, chiefly through rocks, many of which are of the hardest 
description, might seem to an uninformed person to be the principal 
