Address to the Royal Geological Society. 23 
his estimate of the conductivity of surface rocks, availed himself 
of the observations made upon the rocks of Calton Hill, Edin- 
burgh. By calculation applied to the results of observation, he 
determined the thermal conductivity of those rocks while still 
lying im situ, in their natural conditions. Supposing the obser- 
vations to be fairly free from disturbing circumstances, this is 
probably the best mode of investigating this subject. Another 
mode has been followed by the Committee of the British Associa- 
tion appointed for this purpose*, which has its own advantages 
and peculiar capabilities. They have instituted a valuable series 
of laboratory experiments on specimens of various rocks. One 
of their results, which could not be otherwise obtained, is that 
the conductivity of any particular rock, when thoroughly dry, is 
considerably less than its conductivity when moist. Now, acces- 
sible rocks, when we have penetrated a very small depth beneath 
their surface, are moist ; stone when just quarried is, as the stone- 
mason expresses it, “green;’ but the great probability is that at 
a sufficient depth the rocks are dry. We are not now ignoring 
the water concerned in volcanic eruptions, and that which, doubt- 
less, was engaged in metamorphism. It is a common belief that 
it is the internal heat of our earth that keeps the water of the 
earth at, and comparatively near, the surface, and that when our 
earth has cooled down, as the moon evidently has done, that her 
water will sink down into the crust, as has most probably hap- 
pened already with the moon. Here, then, is another circum- 
stance which would make, ceteris paribus, the heated rocks at a 
sufficient depth to be worse conductors of heat than those at the 
surface. 
Thus, then, if we may form an opinion from our limited means 
of judging, the great probability seems to be that the decrease of 
conductivity, owing to the high temperature, and also to the dry- 
ness of the interior rocks, exceeds the increase thereof due to the 
greater density produced by pressure, and that, therefore, the 
conductivity of the interior, neglecting the more inward, probably 
metallic part, is less than that of the superficial parts, which was 
assumed for it by Sir William Thomson. 
We now come to the third of the three conditions assumed by 
Sir William when calculating the rate of the earth’s cooling. The 
* Brit. Assoc. Rept., 1877. 
