834 
ME. W. HOPEIXS’S EXPEEi:viEXTAL EESEAECHES 
and the existence of superficial sedimentary masses of lower condnctirity . In fact, if the 
form of the isothermal surface LM (fig. 3) were established before the formation of the 
sedimentary mass AB, the tendency of that mass, when subsequently formed, would be 
to reduce LM more nearly to parallelism with the external surface ; because the escape 
of heat from M being greater than that from L, the effect would be to lower the surface 
at M relatively to its position at L. 
But if the hypothesis of the particular relation represented in fig. 3 between the 
depression of the isothermal surface at L, and the existence of a sedimentary mass like 
AB, dh-ectly above it, be deemed, as I conceive it must be, far too restricted to be 
admissible, we must consider our hypothesis respecting the non-concentric form of 
the deep isothermal surfaces independently of this additional restrictive hypothesis. 
Now if we do not admit this restriction, there is no reason why the sedimentary mass 
should not be formed above M instead of L (fig. 4). But in such case the quantity of 
Kg. 4. 
D 
_ 
X. 
heat to be transmitted through it would be three times as great as at C, while the corr- 
drrctive power at the former poirrt would only be one-thu'd of that at the latter. Con- 
sequerrtly the rate of increase of temperature in descerrding at D would be nirre times as 
great as that at C. Hence the same hypothesis which might errable rrs to accorrnt for a 
rate of increase of terrestrial temperature, independent of conductive powers, in one 
region, would lead to the almost necessary conclusion that there must be foimd in the 
comparison of these rates of increase, in different regions, deviations from the above law 
far greater than any which have been established by past observations, or ai'e likely to 
be established by future ones. 
I have here supposed (for the sake of speaking of a simple ratioj the conductive power 
of the unstratified mass to be three times that of the sedimentary mass deposited upon 
it. This ratio of three to one may be greater than that actually presented to us in the 
earth’s crust. The actual ratio, however, is probably more than two to one, and abun- 
dantly sufficient, therefore, to justify the above conclusion. 
22. There are other hypotheses, also, which would account for the equality of the rate 
of increase of temperature in descending beneath the surface, independently of the con- 
ductive power of the mass, in the simple case above supposed of a miiform sedimentary 
mass superimposed on the general mass of unstratified rock. It should be remarked, 
however, that in this case, as considered in the preHous article, the equal rate of increase 
of temperature along the vertical lines through C' and D respectively, would only hold 
