836 
ME. W . HOPKIXS’S EXPEEIMEXTAL EESEAECHES 
greatest quantity of heat beneath those strata of which the conductive power is the 
greatest. 
24. In this explanation of the conditions requhred in order that the variations of 
terrestrial temperature as we descend beneath the earth’s surface may be nearly inde- 
pendent of the conductive powers of the masses through which the heat is ti’ansmitted. 
it is assumed that the temperature (tj is the same at the lower surfaces of the two 
groups of strata compared with each other. This, however, is not itself an essential 
corrdition. For when the flow of heat through any mass has become steady^ it is exactly 
the same at every point along the line of transmission, and, therefore (in the case of the 
earth), we may consider any cause generating the heat necessary to maintain this steady 
flow, to act at arry depth without thereby changing the temperatru'e of the superincum- 
berrt mass. The essential condition is that the quantity generated, at whatever depth 
it may be produced, must be irrversely as the conductive power of the mass throrrgh 
which it has to be propagated*. Thus, hr the case of the earth, assu min g terTestraal 
terrrperatrrre to be due to the gerreration of heat in the superficial crarst of the earth, 
then, if we should further assume (for the sake of illustration) that this generation of 
heat orrly takes place in the non-sedimentary urrstratifled portion of the crust, a smaller 
quarrtity of heat must be gerrerated hr those parts of the urrstratifled mass which are srrb- 
jacerrt to sedimerrtary masses of the lower degrees of conducthity, than in those parts 
which extend up to the earth’s external surface. Nor does this appear difficult to con- 
ceive, if, in addition to some such hypothesis as that just mentiorred, we suppose the 
cause producirrg the heat to extend only to comparatively small depths, for, in such case, 
a larger portiorr of the mass capable of having heat generated rvithhr it, worrld be within 
the operatioir of the cause producing that effect, hr that region in which the unstratifled 
rrrass should exterrd up to the external surface. 
25. After the preceding investigations, it appears to me extremely difficrrlt, if rrot 
iirrpossible, to avoid the conclusion that a part at least of the heat now existing in the 
superficial crust of our globe is due to srrperflcial and not to cerrtral carrses. It shorrld 
be remarked, however, that the argumerrt thus afforded is not dh’ectly against the theory 
of a primitive heat, brrt only against the manifestatiorr of the remahrs of such heat as the 
sole cause of the existing terrestrial temperatures at depths beyorrd the dir'ect irrfluence 
of solar heat. The argument in favour of the earth’s original fluidity (a state only con- 
ceivable as the effect of heat), founded on the spheroidal form of the earth, remains im- 
affected. AVhatever cogency it may have been supposed to possess, it possesses still. At 
the same time, all those collateral arguments derived fi’om the existing temperature of 
the earth’s crust, or the climatal changes which we believe to have taken place on its 
* It must be carefully recollected that this conclusion is restricted to those cases in vhich the generation 
of heat takes place entirely beneath the mass in which the increase of temperature is to be independent of the 
conductive power. Any superficial cause would probably generate heat within this mass as well as beneath 
it, so that its temperature w'ould depend partly on heat thus genei’ated within it, and partly on that ti’ans- 
mitted through it from below, in which case the amount of heat generated in a given time need not bear the 
determinate relation to the conductive power stated in the text. 
