But these, and all other extensions of steam power, involve 
consumption of coal, and we may well look with anxiety 
fo our diminishing stock of this precious mineral, which, when 
‘once expended, can never be replaced. It will therefore, be a 
fitting conclusion to this address briefly to review the results ar- 
rived at by the late Royal Commission, of which I was a member, 
as t6 the extent of our available coal and its probable duration. 
I will not trouble you with the vast amount of detailed inform- 
ation collected by the Commissioners as to the extent ofthe British 
coal fields, nor with the elaborate calculations of the quantities of 
coal which those coal fields contain, but I will chiefly direct my 
observations to those points of the inquiry which fall within the 
province of mining and mechanical engineering, and to the broad 
conclusions at which the Commissioners arrived. 
It being well known that a great extent of our coal lies at depths 
greatly exceeding those of our present deepest mines, it was essen- 
tial to the inquiry that the limit of possible depth of working 
should be approximately defined. One of the committees, there- 
fore, into which the Commission was divided, was entrusted with 
this branch of the subject, and having acted in the capacity of 
chairman to that committee, I am especially familiar with its pro- 
ceedings. It fortunately happens that water is never met with in 
large quantities at great depths, and it is easy to exclude it from the 
upper portion of a deep shaft, by the modern process of encasing 
the shaft with cast-iron segments. Nothing, therefore, is to be 
feared on the score of excessive pumping power being required ; 
neither would there be any practical difficulty in drawing coals 
from the utmost depth to which we should have todescend. Steel 
Wire ropes tapering in thickness towards the downward end, would 
not be overstrained by their own weight added to the usual load, 
and even if the depth were carried to such an extreme as to render 
strain on the rope due to its weight a serious difficulty, the 
alternative of drawing at two stages could be adopted. 
With regard to explosive gas it might have been anticipated 
that the greater superincumbent weight upon deep coal would 
Cause more gas to exude, and thereby render the workings more 
fiery, but this does not appear to be the case. On the contrary, 
the evidence given before the committee on this point was to the 
effect that the evolution of gas appeared generally to diminish with 
increase of depth. In short, the only cause which it is necessary 
to consider as limiting the practicable depth of working, is the 
increase of temperature which accompanies increase of depth. 
The rate of this increase of temperature is about 1° F. for every 
feet in depth, starting from 50 feet from the surface, where 
temperature is in this country 50° at all seasons. The ques- 
ions involved in this increase of temperature are, at what depth 
would the air become so heated as to be incompatible with human 
labour, and what means could be adopted to reduce the tem- 
erature of the air in contact with the heated strata. A great 
of interesting evidence was heard by the Commission as to 
the limit of human endurance of high temperature. The natural 
nperature of the human body, or rather of the blood which 
lates through it, is 98°. A higher temperature is the con- 
ion of fever, and the maximum of fever heat appears to be 
105°. Labour appears to be impossible, except for very 
rt intervals when the external conditions are such as to in- 
Crease materially the normal temperature of the blood. The tem- 
perature of the air may be considerably in excess of 98° without 
unduly heating the blood, provided the air be very dry, because 
the rapid evaporation which then takes place from the body keeps 
down the internal temperature; but if the air be humid, this 
‘counter-action does not take place, or not in a sufficient degree, 
ad then the blood absorbs heat from the surrounding medium 
and the condition of fever sets in. Now, in a coal mine, the air 
is never very dry, and is often very moist, and we must, therefore, 
egard a temperature of 98° in a coal mine as the extreme limit 
that could be endured by men performing the work of. miners. 
For my part, I believe this temperature is beyond the limit of 
jossible continuous labour in a mine, and most persons familiar 
with the interior of coal mines will agree with me in thinking that 
even 90° would prove a very distressing temperature, and one 
hich would render the cost of labour much greater than usual. 
However, granting the practicability of working in a coal mine in 
atmosphere at 98°, the next question is, what depth would 
involve that temperature of the air? The depth at which the 
earth would exhibit 2 temperature of 98° would be about 3,000 
feet, but it is a different question at what depth the air circu- 
ng through the mine would acquire that temperature. The air 
g cold when it enters the workings at the bottom of the shafi, 
293 
through which it flows. As it travels along it continues to absorb 
heat, but less rapidly as its own temperature increases, The 
rate of absorption is complicated by the superficial cooling 
of the passages by the contact of the air. This cooling 
action is necessarily greatest near the shaft, where the air is 
coldest, and diminishes by increase of distance, so that both the 
air, and surfaces against which it sweeps, become hotter as the 
length of the air-course is increased. The progress towards com- 
plete assimilation of temperature is much slower in the perma- 
nent air courses than at the working face of the coal, because the 
coal at the face being newly exposed is hotter, and therefore com- 
municates heat more readily to the air. In any case, however, 
the air will eventually acquire the heat due to the depth, if its 
contact with the strata be sufficiently prolonged. It follows, 
therefore, that the temperature of the air in a mine depends on 
the extent of the workings as well as on the depth of the pit. 
But great depth involves extensive workings, because the cost of 
the sinking could only be repaid by working a large area of coal. 
Extremely deep mines will consequently possess doth the con- 
ditions tending to produce a high temperature of the air, and 
unless those conditions can be counteracted by some artificial 
expedient, the air would acquire the temperature of 98°, assumed 
to be the limit of practicable labour at a depth not greatly ex- 
ceeding 3,000 feet. - 
It is a common idea that increase of temperature may be kept 
down to any extent by increase of ventilation, but this opinion 
will not bear examination. In the first place it requires an extra- 
vagant increase of motive power to accelerate the velocity of the 
current of air in any considerable degree, because the re- 
sistance increases in a ratio somewhat exceeding the cube of 
the velocity. In fact, the only way of materially increasing 
the volume of air is by enlarging the sectional area of the 
shafts and air-courses, which would be attended both with diffi- 
culty and expense. Assuming, however, that it would be gene- 
rally practicable to effect a large increase of ventilation under the 
conditions incident to extremely deep mining, it is necessary to 
consider what would be the cooling effect realised by so doing. 
This is a very complex question, because the reduction of tempe- 
rature ia the air increases the emission of heat from the strata, and 
because the rate of absorption is affected, not only by difference 
of temperature, but also by the velocity of the current. 
The uncertainty on the question of the power of air to absorb 
heat when flowing at different velocities and in different volumes 
through heated air-courses, and the difficulty of reasoning out 
any conclusion upon the subject led me to make, for the guidance 
of the committee, a series of experiments in which air was forced, 
in varying quantities, through pipes of different lengths and sizes, 
immersed in hot water, the temperatures being observed at the 
point of emergence. In these experiments the pipes were re- 
garded as representing, on a small scale, the air-courses of a 
deep mine; the hot water being the equivalent of the heated 
strata through which the air would be conveyed. The particulars 
of these experiments will be found in the appendix to the evidence 
taken by the committee, and the results are embodied in tables, 
illustrated by diagrams, which show the progressive heating of 
the air as it travels along the passages, and exhibit the reductions 
of temperature effected by successive increments of the volume 
of air. From these tables and diagrams it will be seen that, 
with short pipes, representing short distances from the shaft, in- 
creased circulation has considerable effect in lowering tempera- 
ture ; but with pipes representing long distances from the shaft, 
the cooling effect of increasing the volume of air becomes insig- 
nificant. The conclusion to which the committee came, as to 
the depth at which coal could be worked, is expressed in the 
following words :—‘‘ The depth at which the temperature of the 
earth would amount to 98° would be about 3000ft. Under the 
long-wall system of working a difference of about 7° appears to 
exist between the temperature of the air and of the strata at the 
working faces ; and this difference represents a further depth of 
420 ft., so that the depth at which the temperature of the air 
would, under present conditions, become equal to the heat of the 
blood, would be about 3,420 ft. Beyond this point the consider- 
ations affecting increase of depth become so speculative, that the 
committee must leave the question in uncertainty ; but they con- 
sider that it may be fairly assumed that a depth of at least 
4,000 ft. could be reached.” : ': 
The committee decline to deal with hypothetical expedients 
for overcoming the difficulties, but they recognised the possibility 
of future discovery and experience counteracting, in some un- 
bsorbs heat with great avidity from the surfaces of the passages | known degree, the effects of heat and humidity in restricting the 
