ON THE CONDUCTIVE POWEES OF YAEIOUS SUBSTANCES. 
8.S5 
to the depth C'C. Beneath C the rate of increase would be only one-third of that above 
C, on account of the greater conductive power below than above that point. The like 
eqiiahty in the rate of increase in the case of art. 17, in which the isothermal surfaces 
are parallel to the outer surface, would result from the supposition that the portion of 
the unstratified mass along the vertical through D should have a conductive power three 
times as great as that of the unstratified mass along the vertical below C. This would 
seem absiu'd ; and, besides, all particular and restricted hypotheses of this kind are liable 
to the objection mentioned in the preceding article. They do not enable us to account 
for the uniform rate of increase of temperature in different localities, unless we adopt 
also the hypothesis of certain necessary relations between the interior structure of the 
unstratified mass beneath, and the deposits of sedimentary beds on its surface — a suppo- 
sition which must be deemed utterly inadmissible. 
AYith respect to merely local de-viations from the prevailing law throughout a region 
of considerable extent, we may observe that it is difficult to refer them to any deep-seated 
source of heat. It would seem much more probable that they are referable to some 
superficial action*. 
On the whole then I cannot avoid the conclusion, that the existence of a central heat 
is not sufficient in itself to account for the phenomena which terrestrial temperatures 
present to us. 
23. Let us now turn to the other case, m which, as expressed in equations (6, 7) 
(art. 15), the variation of temperature in ascending or descending is independent of the 
conductive power, and in which the quantity of heat transmitted through a given hori- 
zontal area is proportional to that power. This, it must be recollected, involves the 
supposition that the two strata whose conductive powers are and /I'., are of great thick- 
ness as compared with that of the other portion of the mass. If, however, instead of 
taking a single stratum of which the conductive powers are respectively and we 
should take groups of strata having mean conductive powers equal to those quantities, 
the same equations will be applicable. Thus if Ave take, for example, groups of strata 
hke those penetrated at the Puits de Grenelle, the coal-shaft at Duckenfield, and the 
other places enumerated above as givuig very nearly the same mean variation of tempe- 
ratm’e along a vertical line, the equations will be applicable, assuming A) approximately 
equal to '■> and it proves that, in such a case, there must be some cause acting to pro- 
duce and maintain a surface of equal temperature at no great depth beneath that to 
which the increase of temperature may proceed according to the same law. At the 
same time, the flow of heat from this isothermal surface must be proportional to the 
conductive power of the superincumbent strata. I have endeavoured to show that this 
cannot be the case when all the heat is transmitted from the central portion of the 
earth, because, in this case, the quantity of heat transmitted must depend on conditions 
existing at comparatively great depths, where they can have little relation to the more 
superficial conditions. Not only, therefore, must there be some other cause generating 
heat to maintain this isothermal surface, but it must also be such as shall generate the 
* Or to water rising from a lower level (see note, page 832). 
