34 Original Articles. [ Jan. 
cations to which the estimates of this great functionary are often sub- 
ject, may well be a warning to all would-be minor prophets not to 
venture on forbidden ground. We feelit, however, necessary to say a 
few words in vindication of what may appear the, somewhat arbitrary, 
limit of depth which we have adopted in the above calculations of our 
coal-resources. The reader will be justified in inquiring why we 
prefer 4,000 feet to 5,000 feet on the one hand, or 3,000 on the other, 
and he is therefore entitled toa reply, though it must be a brief one. 
Taking the latter figure first, we may state at once that this depth 
has already been attained, or very nearly so, in more than one colliery, 
both in our own country and on the Continent,* and no colliery mana- 
ger will maintain that the limit has been here reached. 
With regard to 5,000 feet as a limit of depth the case is otherwise ; 
for we have reason to conclude that supposing this depth to have been 
attained, the temperature, not to speak of other obstacles, would be 
found so high as to forbid the employment of human labour. 
The increase of temperature as we penetrate from the surface, is a 
law which has been established on the evidence of a large number of 
observations in all parts of the world. In our own country very in- 
teresting and careful experiments have been made in several mines ; 
both in the metallic mines of Cornwall, and the coal mines of the North 
of England.t Having on a previous occasion given the experiments 
in detail, the results need only be stated here, and are summarized in 
the following table, together with the temperatures calculated to a depth 
of 4,000 feet. 
Table of Increase of Temperature for Depth. 
; Increase of Increase | Resaltin 
Depth in feet. Pee oe. due te Reel Temparainte 
1,500 21-49 5-0 76:92 
2,000 27°85 6:5 84°85 
2.500 355 8°5 94:00 
3,000 42-14 9°83 102°47 
3,500 49°28 11°66 111°44 
4,000 56°42 13°16 120°08 
In the above table “the temperature of no variation ” adopted, is 
50°5° at a depth of 50 feet from the surface. 
From the foregoing tables it will be seen that even at a depth of 
4,000 feet, a temperature may be expected more than tropical, though 
less than it would be at 5,000 feet, and sufficient, we think, to place 
* One shaft in Belgium, we are assured, is 932 yards in depth. In Saxony, 
there is another upwards of 800 yards; and in the Dukinfield Colliery, the black 
mine has been followed to the depth of 940 yards from the surface. 
+ Experiments made at Rose Bridge Colliery, Wigan, and Dukinfield Colliery, 
Ashton-under-Lyne, and detailed at length in the ‘ Coal-fields of Great Britain,’ 
pp. 223-232. The latter were first published by Mr. W. Hopkins, F.R.S., in the 
* Philosophical Transactions,’ vol. exlvii. 
