92 SEVENTH REPORT—1837. 
proved to be the case by geodetic observations and calculations ; and 
that this fluidity has been at least partially preserved, may be inferred 
from the phenomena of Basaltic Dykes evidently protruded since the 
deposition of the strata they traverse. The inquiry, therefore, into the 
temperature of the earth, at various epochs, is closely connected with 
that into the mode of its cooling. In entering on this it must be 
remembered, that though natural causes are permanent, the conditions 
under which they act may not be so, and hence also the effects may 
vary. The processes, for instance, will be different, according as the 
cooling is effected, in a fluid or in a solid body. If in a fluid, the re- 
frigeration will be by circulation, and there will be a period at which 
that circulation must stop ; but whether beginning by the surface or 
centre, may admit of doubt; for if the temperature of the surface be 
the least, the pressure is there also the least ; and if the temperature 
of the centre be the greatest, the pressure is there also the greatest ; 
and henee, as the expansive force of heat, and the pressure of gravita- 
tion are two opposing forces, the one resisting, and the other promo- 
ting solidification, that solidification will commence at the surface or at 
the centre, according as the one or the other shall preponderate ; but 
at present evidence is insufficient to decide for one or the other; whilst 
the process of refrigeration was effected by circulation, the upper, or 
heavier (being cooled) particles would descend, and the lower, or hotter, 
ascend; but at the instant when the process of circulation ceased, 
the further cooling would be effected by conduction. According, 
therefore, to the different processes assumed as having operated the 
change of temperature of the earth, the earth may be viewed as, 
Ist. Entirely solid, and cooling by conduction. 
Qnd. As having solidified first at the centre, the pressure there over- 
coming the expansive force ; and next, at the surface, the depression of 
temperature there more than compensating for the low pressure, and 
thus placing a fluid annulus between a solid nucleus and a solid crust ; 
and, 
3rd. As having solidified first at the surface ; a solid crust surround- 
ing a fluid centre. But as the laws by which consolidation and refrige- 
ration are regulated are as yet very imperfectly understood, the ques- 
tion is still one of doubt. 
Mr. Hopkins further remarked, that the increase of temperature on 
descending into the earth was apparently established as a fact; the 
question was, whether that increase was due to central heat, and the 
evidence is yet insufficient to solve it. M. Poisson, indeed, considers 
that it is almost impossible that heat should so increase to the centre, 
for at the temperature, which would be the result, the expansive force 
would be so great that no crust could resist it: and he has, therefore, 
advanced the hypothesis, that such temperature is not due to central 
heat, but that the whole solar system is now passing from a hotter to a 
colder portion of space, and is consequently losing the heat it had 
acquired in its passage; an hypothesis which has, however, met with 
powerful opponents. 
