310 PROCEEDINGS OF SECTION C. 
‘A great many determinations have been made of this rate 
of increase at various times in Europe and in America, for, 
as regards Europe, it would be difficult to say when the fact 
that there was a rise in temperature was first discovered, 
the matter being so obvious as soon as mine operations were 
of any depth, not to speak of the evidence afforded by warm 
springs; but aninspection of theresultsof such measurement 
very soon showed, either that the problem of taking the tem- 
perature of rock was not quite so simple as it looked, or else 
that there was a considerable divergency in the rate of in- 
crease in different districts. A repetition of the measure- 
ment in the same districts by other and later workers, as 
opportunity served, has clearly shown that many of the 
results that were first put forward, and are even now in 
text-books, were inaccurate, but that, even when these in- 
accuracies were eliminated, there still remain, as might be 
expected, local divergencies in the rate. The measurement 
reminds one of the more marked instance afforded by the 
determination of the composition of the atmosphere, where 
the larger differences in composition noted by early and 
unskilful experimenters disappeared when more attention 
was given to methods employed, so that finally only differ- 
ences of a smaller order of magnitude were proved to exist, 
unless there were manifestly local and accidental reasons for 
the difference also to be noted. 
One must, even to-day, refer to the methods to be 
employed. There are two main ways by which we can get 
the measuring-instrument down to the place at which the 
temperature is to be taken. Either we can make use of a 
borehole made by a diamond-drill or by a percussion-drill, 
as for the purposes of exploration, or else we must take 
advantage of a mining shaft, by which we can ourselves 
approach very near to the point of observation, and take 
our reading. 
The case where a vertical borehole is used has a few little 
difficulties of its own. Im the first case, the act of sinking 
the borehole often involves local alteration in the tempera- 
ture of the walls of the bore, for there is actual work done 
on the rock in order to bring about the removal of the latter. 
There is a disturbancein the temperature by the insertion of 
the iron lining-tubes, the material of which is a far better con- 
ductor of heat than the rock. The iron boring-rods that 
must be inserted in order to carry the instruments introduce 
a disturbance of the same kind; but all these disturbances 
are small, and, whilst an allowance of ample time before a 
reading is taken will eliminate most of them, the loss due 
