464 Our Coal Supply and our Prosperity. ' [Oct., 
the roof. This may be, when the strata are unbroken by faults or 
fissures over wide areas ; an exceptional state of things. In such a 
case Mr. Vivian’s analogy of “the small hole in the wall ”* holds 
good, but if the cohesion of the beds is destroyed by ruptures (a 
very common occurrence in the form of joints and faults), we may 
expect the weight of the strata to exert an irresistible effect. We 
have been strongly impressed with the force of this reasoning, by 
a case which occurred in Dukinfield colliery, as stated by the 
manager about two years since. In the workings of the coal, 
42 feet in thickness, at a depth of 2,500 feet from the surface, the 
pressure was found powerful enough to crush in cireular arches of 
brick-work, four feet in thickness; and in one instance a pillar of 
cast-iron, 12 inches square, 44 feet in height, and supporting a roof 
of only 7 feet square, was snapt in twain. 
Notwithstanding these considerations, we cannot but feel that 
in questions of this kind experience is likely to be the only safe 
euide, and we think this is one of the points on which the Royal 
Commissioners ought to ascertain the experience of the colliery 
managers and viewers of the deeper mines throughout the king- 
dom. Admitting, for the moment, the possibility that neither 
temperature nor pressure is likely to be found an insuperable 
obstacle even at the depth of 4,000 feet, we may confidently 
assume that only coal-seams of superior quality and thickness, 
capable of being mined economically, will be followed to this or 
any greater depth. The question of outlay of capital and additional 
cost per ton due to increase of depth, becomes formidable, and as 
Mr. Jevons has shown, this will be the last court of appeal in 
all questions of mining. To open out a colliery at the present 
day to a depth of 600 or 700 yards, cannot be accomplished under 
an outlay of 100,000/., which sum has to be recouped in a term 
of twenty-five years or so, and to bear interest at the rate of 
10 per cent. in order to form a successful speculation, and this can 
only be done where the seam is of good quality, proper thickness, 
and comparatively free from accidental irregularities, such as rock- 
faults, dykes, and “ horse-backs.” 
When therefore the outlay comes to be nearly a quarter of a 
million, as it assuredly will be when the depth is 1,000 yards and 
upwards, instead of 600 or 700, we may feel sure that only seams 
of superior excellence will justify such an outlay. It is therefore 
inconvenient to adopt any sharply-defined limit of depth, as we may 
~ easily conceive the case of a seam of coal of such value, that it 
might be followed even to a greater depth than 4,000, while we 
may safely conclude that there are very few seams in any coal-field 
which will eventually justify such an effort. Still, admitting this, 
* “The fact was, that you might make a small hole in a wall and yet not 
cause the wall to tumble down.’—His ‘Speech. Ridgway. 
