Address to the Royal Geological Society. 19 
many scores of miles, as Sir William believes, or whether it 
continues to increase to a very much greater depth. 
To proceed now to the second of the three conditions assumed 
by Sir W. Thomson when calculating the rate of the earth’s 
cooling; viz., the probable heat-transmitting power of the body of 
the globe, or at least of the more outward parts of the body of the 
globe. It is our interest to keep this down as much as possible. 
A more rapid rise of temperature in descending into the earth, to 
the depth of which man has knowledge, might be due to any of 
the following circumstances, or to some of them conjointly :—1. 
The shortness of the time of cooling of the earth. 2. The lower 
conductivity of the rocks to that depth, as compared with surface 
rocks. 3. The higher conductivity of the rocks below that depth. 
Sir William accounts for the rapid rise of underground tempera- 
ture, in depth, by the first of these, the shortness of the earth’s 
cooling period. We would submit that the second, also, may ke 
concerned ; if it be so, the argument for the first is weakened, 
which is all that we desire. It is of relatively small importance 
to us whether the third obtain or not. 
Sir William, in order to have something to go upon, has assumed 
(not that he believes it) the absolute thermal conductivity and 
the thermal capacity of the globe to be that of average surface 
rocks, At first sight it might seem probable that this may not be 
much beside the mark, for there are two principal influences which 
tend in opposite directions in their effect on the result. The 
tendency of the rise of temperature which obtains as we go down 
is, ceteris paribus, to diminish the heat transmitting powers of the 
materials below. The tendency of the increase of density is to 
increase the heat transmitting powers of those materials. Do 
these opposing tendencies probably balance each other approxi- 
mately ? or, if not, which of the two more probably prevails 
over the other ? 
Of course we now move with very great caution, remembering 
that in such a question as this, we cannot argue positively from 
what has been ascertained merely within the limited range of 
conditions accessible to the physicist in his laboratory. As 
Sir William himself says “we are very ignorant of the effects 
of high temperatures in altering the conductivities and specific 
heats of rocks, and as to their latent heat of fusion.” But the 
ScieN, Proc. R.D.S. Vou. 11, Pt, 1. C2 
