15S 
ME. EOBEET MALLET ON VOLCANIC ENEEGT. 
boring the rate of increment has been found to vary at different parts of the descent. A 
large proportion of the observed temperatures certainly fall within the limits of l°Fahr. 
in 30 feet and in GO or 70 feet of descent. 
The most complete and valuable collection and discussion of all the observations on 
record up to June 1836 which the writer has met with is to be found in a rare and scarcely 
known Inaugural Dissertation for the degree of Doctor of the Rheno-Trajectine Academy 
(4to, 101 pp., Muller, Amsterdam, 1836), entitled “ Disputatio Physica Inaugularis de 
Calore Telluris infra superficiem augescente,” by A. Vrolik. The careful and laborious 
author places the following amongst his general conclusions : — “ Variarum observationum 
autem eventus, acleo inter se discrepare ut certarn incrementi legem pro unaquaque 
regione nondum statuere possimus.” He believes it proved that in general the rate of 
increment is greater in plains and valleys than in mountains. 
41. These conclusions have been sustained by the later observations made since 1836. 
De la Beche, Herscilel, and Babbage enunciated more or less clearly a connexion 
between the increment of hypogeal heat and the conductivity of the superposed strata. 
That was afterwards examined carefully by Hopkins in one of his most valuable 
memoirs, that “ On the Conductive Powers of various Substances, &c.” (Phil. Trans, 
vol. cxlvii., 1857). Towards the conclusion he remarks, “ On the whole, then, I cannot 
avoid the conclusion that the existence of a central heat is not sufficient in itself to 
account for all the phenomena which terrestrial temperatures present to us” (p. 835); 
that is to say, that a cooling globe, together with such effects as his experiments on 
conductivity of its materials warrant, are still insufficient to account for the observed 
discrepancies in hypogeal increment of heat. 
42. One great source (if not the only one) for the unknown residual phenomena thus 
distinctly indicated by Hopkins we hope presently to point out, and to prove its inti- 
mate connexion with existing volcanic phenomena. 
43. Our present knowledge of hypogeal temperature, while thus and so far insufficient 
to sustain any very minute conclusions based upon annual absolute loss of heat by 
our earth, seems surely to support the fact that our earth is a cooling globe ; while 
astronomical analogies and considerations of its figure seem equally to warrant 
the presumption that it has been a cooling globe at all times from a state of fusion ; 
and if so, that from that period and up to the present it has been on the whole 
a contracting globe. 
These are all the conditions we require to admit for the conclusions to be obtained 
by considering the sequence of the phenomena that have followed on refrigeration. 
44. It may be remarked, in passing, that the general credence of geologists in the 
cooling at all of our globe has been more or less disturbed by the celebrated memoir 
of Laplace (Mecanique Celeste, tom. v. cap. iv. p. 72 &c.) “ On the Cooling of the 
Earth as affecting the length of the Day,” in which it has been taken for granted, by 
most geologists at least, that the great mathematician has irrefutably proved that our 
globe has not cooled sensibly for the last 2000 years. 
45. With due reverence for the intellectual sovereignty of Laplace, the writer ventures 
