i86 On Under ground Temperature. [April, 
if it were possible to reach such depths as the geologist ven- 
tured to speak of, the rate, on Sir W. Thomson’s hypothe- 
sis, would be found to be sensibly diminishing, and, even 
in the centre of the earth, a greater temperature need not 
exist than what he expedted to find at 60 miles. The 
greater the value we assign to the presumed age of the 
world, the further down we should have to dig before we 
should find the rate sensibly diminishing. Some interesting 
particulars upon the relation between different assumed 
values for the world’s age, and the depths at which it might 
be practicable to detedt a diminution in the rate of increase 
of temperature, will be found in the address referred to. 
But the conclusion at which, upon the whole, we must 
arrive is that — on the supposition that the heat of the earth’s 
crust comes up from below (and is not generated, as Prof. 
Mohr supposed, within itself) — no mine nor bore-hole is 
ever likely to be put down deep enough to solve the question 
whether the earth is solid from surface to centre, or not so. 
If ever it could be conclusively proved, by observation, that 
the rate began to diminish, that would be an argument 
that the earth is a solid cooling by conduction from its 
entire mass. But the shortest age which can be assigned to 
the world’s history is too long to allow us to expedt that 
such a depth will ever be reached. 
The objedt of the portion of this article which relates to 
the observations at Sperenberg has been to demonstrate 
that the diminution in the rate of increase supposed to exist 
at that place was only apparent, and could be accounted for 
without assuming any such diminution of the rate to exist in 
the rock itself. 
As a fadt, however, the geologist has to deal only with the 
surface phenomena of the globe. It is a matter beyond his 
immediate province to enquire what may be the condition 
of the earth’s interior, except to just such an extent as that 
must influence the surface conditions. Accordingly he is 
ready to accept from the physicist such conclusions as can 
be arrived at solely by methods outside his own province of 
study. But he asks the physicist, in return, not to ignore 
the well-ascertained conclusions of geology, nor to suppose 
that the mistake must certainly be on the geological side if 
they do not square entirely with his own results. The 
writer would beg to be allowed to submit that there are 
some things which do not well agree with Sir W. Thomson’s 
convidtion that the earth became solid throughout, and has 
since cooled as a solid ; for such a supposition will not 
account for the phenomena of surface inequalities, as geolo- 
gists have almost unanimously agreed to interpret them. 
