ON THE CONDUCTIVE POWEE8 OF VAEIOUS SUBSTANCES. 
837 
surface, are deprived, I conceive, of a great part of their Aveight. Moreover, admitting 
only a part of existing terrestrial heat to be due to supeidicial causes, the flow of heat 
from the earth’s central portions must be less by that amount than if the whole flow 
were due to central heat. Consequently the rate of increase of temperature due to the 
flow of central heat must be proportionally diminished, and the depth at which we 
should arrive at the temperature of fusion, proportionally increased. The conclusion, 
therefore, that the earth’s solid crust is so thin as many geologists have believed it to 
be, as well as those theories resting upon that conclusion — whether of volcanic action, 
or of elevation and depression of the earth’s surface, at least in more recent geological 
times — must be in a great degree invalidated. But on these points I may have some 
further remarks to make in my next communication. At present it is not my object to 
carry these speculations beyond the point at which we are now arrived. 
§ III. Descrijition of the Apparatus for determining Conductive Powers, and 
the mode of conducting the Experiments. 
26. I now proceed to an account of the experiments, the general results of which 
have been given, and to the description of the apparatus by which they were made. 
Fig. 5. Fig. 6. 
Fig. 5 represents the different parts of the apparatus separated from each other. In 
fig. 6 they are represented as fitted into each other preparatory to an experiment. F is 
5 R 
MDCCCLVII. 
