1 877.] On Underground Temperature . H75 
variable heat-condudting power in the layers successively 
penetrated. The observations in the bore were taken with 
much precaution, under the direction of Herr Eduard 
Dunker, Inspector of Mines. A resume of his paper upon 
them will be found in “ Nature ” (No. 376, for January 11 of 
the present year) as contained in the “ Ninth Report of the 
British Association Committee on Underground Tempera- 
ture.” In No. 312 of the same publication (1875) there had 
previously appeared a notice of a contribution, by Prof. 
Mohr, of Bonn, to the “ Neues Jahrbuch fiir Mineralogie ” 
(1875), in which that gentleman had commented upon the 
law of increase which he supposed to he deducible from the 
observations made in this bore-hole. The result at which 
he arrived was remarkable enough, and if it could not have 
been explained away would have justified the conclusion 
which he drew from it, which was that the source of heat 
within the crust of the earth was situated within the crust 
itself, for that after a certain depth was reached — which he 
put at 5170 feet — the increase would he nil. However, in 
the Report of the British Association just published, it is 
explained how this result had been arrived at, and it is dis- 
tinctly shown that Prof. Mohr’s conclusion was in reality 
not based upon the original observations themselves, but 
that it arose from the form of an empirical mathematical 
formula which Dunker had assumed to express the law ; so 
that the law which brought the increase of heat to nil at 
5170 feet was of Bunker’s making, and not Dame Nature’s. 
In faCt it had been implicitly assumed that the increase 
would come to an end, and all that Mohr did was to find 
out at what depth it would do so, supposing that assump- 
tion true. A better instance can hardly be required to show 
the extreme caution necessary in accepting the conclusions 
of philosophers when they are, on the face of them, hetero- 
dox. The safe attitude of the mind in such a case is to 
suspeCt that there must be some mistake, and the duty of 
the competent is to try and find it out. 
This bore-hole, from the favourable circumstances already 
referred to, and the care with which the observations were 
made, deserves full consideration. The temperatures were 
taken in two manners. In one set of observations the tem- 
perature of the water in the bore-hole was observed with a 
suitable thermometer ; in the other an apparatus called a 
geo-thermometer was lowered, which cut off a portion of 
water from that above and below it by two disks or bags. 
A thermometer was enclosed in the space between the 
disks, and, after the thermometer had been down not less 
