nds to fix itself in our minds in
exaggerated proportions to its true place in the cosmic machine; and, in
fact, nearly all who have sought to expound its nature and mode of
origination have occupied themselves far too exclusively with describing
and theorising upon the strange and varied phenomena which the volcanic
cone itself and its eruptions present, and too often, in the splendour
and variety of these, have very much lost sight of what ought to be the
centre-point of all such studies, namely, to arrive at some sound
knowledge of what is the _primum mobile_ of all these wonderful efforts.
Nor has the distinction been very clearly seen between the main
phenomena presented at and about volcanic active mouths, which can be
employed to elucidate the nature of the causation at work far below, and
those most varied and curious, and in other respects most pregnant and
instructive phenomena, mechanical and chemical, which are called into
action in and by the ejected matter of the volcanic cone after its
ejection. It can help us but little or very indirectly, in getting at a
true conception of the nature and source of the heat itself of the
Volcano, to examine, for example, all the curious circumstances that are
seen in the movements and changes in the lava that has already flowed
from its mouth; but it would be of great importance if we can ascertain,
by any form of observation around the cone, from what depth it has come,
or at what depth the igneous origin lies.

The physician, endeavouring to ascertain the real nature of small-pox or
measles, will scarcely make much progress who, however curiously or
minutely, confines his attention to the pustules that he sees upon the
skin.

Yet the Volcano, or rather all volcanic activity as now operative upon
our globe, is, as it were, an experiment of Nature's own perpetually
going on before us, the results of which, if well chosen--that is, as
Bacon says, by keeping to the main and neglecting the accidents--can,
when colligated and correctly reasoned upon, in relation to our planet
as a whole, give us the key to the enigma of terrestrial Vulcanicity in
its most general sense, and at every epoch of our world's geognostic
history, and show us its true place and use in the cosmical machine. Let
us glance at the history of past speculation on this subject, from which
so little real knowledge is to be derived, and then at the salient facts
of Vulcanology as now seen upon our earth, an