eo —-~ wee ee eee eee 
THERMAL CONDUCTIVITIES OF ROCKS 305 
It has been found that the remaining Bessel functions will require at 
least two further volumes, the first of which, containing about 300 pages, 
will tabulate the higher integral orders (up to nm = 20) of the four functions 
Fn» Ynys In, Ky. The calculations for 7,, (x), up tox = 25, are now practically 
complete ; work on the other functions is also in hand. 
Reappointment.—The Committee desires reappointment, with a grant of 
£200, the greater part of which would be devoted to calculations for the 
next volume of Bessel functions. 
THERMAL CONDUCTIVITIES OF ROCKS. 
Report of Committee appointed to investigate the direct determination of the 
Thermal Conductivities of Rocks in mines or borings where the tempera- 
ture gradient has been, or is likely to be, measured (Dr. EzER GRIFFITHS, 
F.R.S., Convener; Dr. E. C. Butiarp, Dr. H. Jerrreys, F.R.S., 
Mr. E. M. Anperson, Prof. W. G. FEARNSIDES, F.R.S., Prof. A. 
Ho.mes, Dr. D. W. Purutrs, Dr. J. H. J. Poote, Mr. W. CAMPBELL 
SMITH). 
THE rate of outflow of heat per unit area of the earth’s surface is the product 
of the thermal conductivity and the vertical temperature gradient, and can 
therefore be found if these two quantities are known. Different determina- 
tions by experiment and observation have, however, given substantially 
different results, and the variation requires explanation. ‘The rate of out- 
flow of heat is an important geophysical quantity connected with the amount 
of radioactivity within the earth, but has been determined in practice by 
taking scattered measurements, averaged by methods that are little better 
than a guess. Unfortunately, there seems to be no case where the tempera- 
ture gradient and the conductivity have actually been measured in the same 
rocks. ‘The possibility that there is a systematic difference between the 
rocks whose conductivities have been ascertained, and those where the 
gradient has been measured, therefore remains untested, and so does the 
possibility of a real variation of the heat outflow from place to place. 
The work of the Committee during this year has been wholly exploratory. 
Two meetings have been held, and a long report on the existing determina- 
tions of temperature gradient has been drawn up by Dr. D. W. Phillips. 
It appears that much work has recently been done in the United States, 
Central Europe and Russia. ‘Temperature gradients appear to be related 
to geological structure, and have therefore been used to some extent in 
prospecting. ‘This is reasonable, because theoretically a body of high con- 
ductivity near the surface should make an easy path for the heat, and a high 
gradient would be found above it. This is a three-dimensional effect, 
which will arise only for inequalities with horizontal extents small in com- 
parison with the depth of the main sources of heat, and near the edges of 
larger ones. Some other anomalies have been traced to the presence of 
intrusions that have not yet had time to cool down, and others to the dis- 
turbance produced by the last glacial period. These variations will have to 
be allowed for in any accurate determination of the heat flow at great depths. 
The Committee desires to send forward the following recommendations 
to the Committees of Sections A and C: 
(1) Casual collection and examination of rocks to determine conductivities 
can add little to what is already known. The collection of material for 
study should be done while mines or borings are actually being made, and 
