ON THE CONDUCTIVE POWEES OF VAEIOUS SUBSTANCES. 
829 
igneous rocks I have tried are of high, but not very variable conductivity ; and from all 
we can know by observation, we have no reason to suppose that the unstratified portion 
of the earth’s crust presents any variation of general structure which would be likely to 
influence very materially its conductivity in different localities. For these reasons we 
may, I conceive, confidently assume that for any such depth as fifty miles, we shall have 
K'-K’ 
very approximately. In this case equation (9.) becomes 
dz^ 
i. e. the rates of change of temperature in ascending or descending will be very nearly in 
the inverse ratio of the conductive powers. This interpretation applies immediately to 
two strata in the two separate groups of strata ; but it manifestly applies also to any two 
strata in the same group ; since for each group the flow of heat is constant, and therefore 
for every stratum 
const. 
We also observe, that when 
H'_ H 
K'“K’ 
as above supposed, equation (8.) shows that the flow of heat is the same in both groups 
of strata. 
17. The result at which we have arrived respecting the rate of change of temperature 
in ascending or descending through different strata, is of so much importance in testing 
the theory of terrestrial temperature which attributes the actual temperature of the 
earth’s crust entirely to central heat, as we are now supposing, that it may be worth 
while to elucidate the above general conclusions by a particular and simple case. 
Let us suppose, then, a single stratum AB (fig. 2) of sedimentary matter of about 100 
Fig. 2. 
A w _a) 
miles in radius as above supposed, superimposed on the unstratified mass (C) of the 
earth’s crust, which is supposed to remain uncovered with sedimentary beds in all the 
neighbouring region. Let us also suppose the thickness of our sedimentary bed to be 
about 1000 feet, and its conductive power to be one-third of that of the unstratified 
mass. Now let us first conceive the entire absence of the stratified mass. The tempe- 
rature at the depth of 1000 feet, according to the actual observed rate of increase* of 
temperature as we descend (about 1°Fahe. for every 60 feet), would be nearly 
=T+16°; 
5q 
MDCCCLVII. 
