[eee ey ee es 
ON SEISMOLOGICAL INVESTIGATIONS. 219 
a sufficiently northern position. Such traces were not found,at Bombay and the greater 
sensitiveness at Calcutta was attributed to the less stable condition of the earth’s 
surface in that neighbourhood ; a mercury trough there is so unsteady as to be useless 
for giving a vertical line until the mercury is amalgamated; and the occurrence of 
‘Barisal guns’* suggests abnormal geological conditions. On the ground of economy, 
the officers in charge of meteorological work at Calcutta have always had serious 
teaching responsibility in the Calcutta University, and my appeals to them to compare 
the period of the ground oscillations indicated by the mercury trough under the 
transit instrument with that of the microseisms have not borne fruit. 
The oncoming of the monsoon is in some years a gradual development, but in 
characteristic seasons the ‘ burst’ is associated with gales and rough weather at sea ; 
so that there is, I think, little doubt that the installation of the Milne-Shaw at Bombay 
promises not only to throw light upon the tremors produced by waves on shore, but 
also to afford indications of considerable value some days before the breaking of the 
monsoon. 
~The Earth’s Thermai State. 
(By Dr. Harnoitp Jerrreys, F.R.S.) 
Since it was discovered that rocks contain a considerable amount of radioactive 
material, which is continually generating heat, the Kelvin theory of the cooling of the 
earth has had to undergo much revision to allow for it. The amount is such that a 
layer of granite, something like 30 km. thick, would produce as much heat as is shown 
to be leaking out of the earth by the observed increase of temperature downwards. 
If there is more radioactive material than this, the interior of the earth must be under- 
going continuous heating. 
Cosmogony indicates that a cooling earth is more likely than a heating one, for 
it is very difficult to avoid the conclusion that the earth was formerly fiuid. An 
additional argument, of much weight, was given by Dr. Arthur Holmes in a series of 
important papers in the Geological Magazine for 1915-16. Supposing the temperature 
now to be everywhere steady, and accordingly the depth of the granitic layer to be just 
enough for its radioactivity to account for the observed outflow of heat, he found the 
temperature at the base of the layer and at all greater depths to be about 300° C., 
which is hopelessly inconsistent with the phenomena of vulcanism. No reasonable 
modification of the hypothesis was found capable of reconciling the facts, and the 
hypothesis of a rising temperature made matters worse. 
Holmes therefore adopted the hypothesis that radioactivity falls off with depth so 
rapidly that it is insignificant throughout most of the crust; there is independent 
evidence, both geological and chemical, for believing that this is true. With reasonable 
assumptions as to the earth’s state just after solidification, the present rate of outflow 
of heat then gave an equation to determine (practically) the total amount of radio- 
active material in the crust, and hence the cooling at all depths. The results obtained 
have undergone some modification since that time, partly by the present writer and 
partly by L. H. Adams, whose discussion is the most complete yet published, but the 
alterations have not been great. They imply a thickness of the granitic layer of 
the crust of 13 to 20 km., according to the precise assumptions made. Without 
further special assumptions, this theory leads readily to an estimate of the thermal 
contraction of the interior agreeing roughly with that needed to account for the folding 
in known mountain ranges, and to one of the horizontal extent of isostatic anomalies 
of uniform sign which also agrees reasonably with observation. 
An alternative view has been propounded in several recent papers by Professor 
J. Joly, who postulates that great radioactivity extends to a much greater depth than 
is above indicated. The difficulty, that this would imply a rate of increase of tempera- 
ture downwards much greater than is observed, is met by supposing that the radio- 
activity produces fusion, and that the formation of a fluid layer is recurrent, leading 
to a periodic variation of temperature. The present rate of outflow of heat is then to 
be regarded as much less than the average. Such a periodicity would explain the 
existence of tension phenomena in the crust, which have not been satisfactorily 
explained on the former theory, though they are not inconsistent with it. But there 
seems to be a fundamental objection to the postulate of periodicity. The problem is 
one of heat conduction under the influence of a steady supply of heat, with free radia- 
tion from the surface. It is inherent in the nature of heat conduction in solids that in 
1 This is the name given to subterranean sounds as of distant artillery heard in 
Lower Bengal—also in Holland and in Australia. 
