part of Europe, namely, at the Quicksilver Mines of Idria, where the metallic sulphurets likewise occur, and as it is well known that a considerable degree of heat is given out by such substances during decomposition, I cannot help believing that the temperature was, in those cases, affected by their presence. I may add, that even respecting the Cornish mines, the statements given are very contradictory, for it is not long since there appeared an account by a Cornish gentleman of some trials, from which it would appear, that the heat below was in several instances no greater than that of the external air.

Now, as he justly observes, a single well-ascertained case of this kind ought to outweigh a hundred observations which shew an increase, since it is far more likely that the temperature of the earth should be raised above its natural standard by local causes, than that it should have been reduced below it.

Neither does it seem very consistent with the ordinary progress of nature to suppose, that the earth is gradually sinking in temperature, so that it will in time become unfit for the abode of the present race of animals, notwithstanding the warmth communicated to it by the sun.

It is true that the consideration of final causes ought not to be suffered to interfere with proofs of a more positive kind, but it may surely be introduced as an element into the calculation, when the utmost we can pretend to have arrived at, are probabilities.

Yet though there seems to be no reason for supposing the earth's temperature to be at present undergoing diminution, I am ready to allow that the presumption arising from a fair review of the 'phænomena is rather favourable than otherwise to the notion of a central heat;-all I object to is the bringing forwards, what ought only to rank in the light of an hypothesis, as the basis of elaborate mathematical investigations.

For my own part, however seductive it may be to the imagination to explain on some one broad principle the phænomena of our globe, and to lay down the great ends which volcanos are calculated to serve in the economy of nature, I think it

more consistent with sound philosophy to limit myself, to those effects which have obviously been produced by their action, and to those final causes of their existence, which may be presumed from phænomena which we ourselves witness.

The former of these inquiries has already been insisted upon, and the occurrence of basalts in every class of rocks, under circumstances which establish igneous action, indi. cates that volcanos have existed almost from the commencement of our globe.

With respect to the latter point, I shall only remark, that whatever may have been the end, for the sake of which the accumulation of inflammable materials in the interior of our globe was ordained, their existence there, under circumstances which admitted of their undergoing from time to time inflammation, rendered the production of volcanos not only a natural consequence, but even an useful provision.

They are the chimneys, or rather the safety-valves, by which the elastic matters are permitted to discharge themselves, without causing too great a strain upon the superficial strata.

Where they do not exist, they give place to a visitation of a much more destructive nature; for those who have experienced a volcano and an earthquake will readily testify, that the consequences of the one are by no comparison lighter than those of the latter.

The same country is indeed often exposed to this double calamity, but that the existence of the volcano is even there a source of good, appears from the fact, that the most terrible effects are felt at a certain distance from the orifice, although the focus of the action is probably not far removed from the latter.

The agitations, which took place during six years at Lancerote, likewise shew, how much more destructive the effects of subterranean fire appear to be, where no permanent vent is established.

Thus far we have proceeded on solid grounds, but if we are willing to push the enquiry farther, and to speculate on

the other ends which volcanos may be intended to answer, it may perhaps not be too bold an hypothesis, when we consider their general distribution, to imagine that they are among the means which nature employs, for encreasing the extent of dry land in proportion to that of the ocean.

That such is the tendency of the processes daily taking place, appears from various considerations, and from none more remarkably than from the formation of coral reefs, a cause of increase to the quantity of dry land, with which the destroying agencies that are also at work have nothing to compete.

In speaking of the Canary Islands I observed, that volcanic processes seem much more frequently to have elevated, than to have submerged, tracts of country; and if we consider, that coral reefs are mostly founded on shoals caused by volcanic matter that has been thrown up, a sort of consistency will appear in this instance to exist in the arrangements of nature, which leads to the belief, that fire and water are both working together to a common end, and that end, the preparation of a larger portion of the earth's surface for the reception of the higher classes of animals.

There may be something fanciful in what I am now going to suggest, with regard to another end which volcanos may be conjectured to fulfil; yet if there be any truth in the idea, that the pressure of the ocean would be constantly forcing a certain portion of its waters through fissures into the interior of the earth, it would seem that there ought to be some compensating process, by which the ratio between the sea and land might be preserved unaltered.

This would perhaps be afforded by the action of volcanos, which restores to the surface just as much water as has been admitted to the spots at which the process is going on; for though the first effect of the action is to decompose that fluid into its constituents, yet the immediate consequence is, as we have seen, the disengagement of a large volume of sulphuretted hydrogen and sulphurous acid gases; so that by the action either of the latter fluid, or of atmospheric air

upon the former, the whole of the hydrogen of the water, sooner or later, becomes re-united with oxygen. ́ This indeed is one cause of the quantity of steam given out from the craters of all burning mountains.

The products of the volcanic action also, though, from the individual mischief they occasion, they can hardly be viewed by the inhabitants of the country overspread by them in any other light, than as serious present calamities, do not nevertheless deserve to be considered as permanent or unmixed evils.

It is true, that there is something gloomy and depressing in the contemplation of a volcanic mountain, when we consider the cities it has overwhelmed, the fields it has reduced to desolation.

Yet if we do not adopt the notion once so prevalent with respect to the speedy dissolution of the globe, if we take up the more pleasing, as well as, I conceive, the more probable opinion, that a world, which required so many ages to prepare it for the accommodation of its present inhabitants, is destined for many ages more to afford them a suitable abode; there is then something consolatory in the reflexion, that the very lava, which for so long a period has spread the most hopeless sterility over the ground it traverses, in process of time crumbles into the richest of soils; and that, if we take the case of the neighbourhood of Naples as the volcanic district with which we are best acquainted, the experience of what has happened before justifies a belief, that the inflammable materials which supply the fires of Vesuvius will ultimately be expended, and that the mountain may at some future period return to the fertile condition, which Martial describes as belonging to it, when its heights were covered with vineyards, and the very spots surrounding the actual crater were considered the favourite resort of the Gods.

Hic est pampineis viridis Vesuvius umbris,
Sparserat hic madidos nobilis uva lacus.

Hæc loca, quam Nysæ colles, plus Bacchus amavit,
Hoc nuper Satyri monte dedere choros,

Hæc Veneris sedes, Lacedæmone gratior illi,
Hic locus Herculeo nomine clarus erat.

ADDITIONAL NOTES.

Note to page 145.

On the Picture found at Herculaneum.

I find that the statement, given on Newspaper authority, respecting the antient picture of Vesuvius, lately dug up, is contradicted by the Editor of the Giornale delle due Sicilie.

Note to page 276.

On the neighbourhood of the Red Sea:

The Ichthyophagi of the environs of Ptolemais, in the Thebaid, preserved in the time of Agatharcides, the remembrance of an earthquake, during which the sea was left dry-See Diod. Siculus, 1. iii. c. 1.

That the following phænomenon also, the knowledge of which I owe to my friend Mr. Gray, of University College, Oxford, is connected with any thing volcanic, may be uncertain; but as it is curious in itself, I shall insert his account of it, which I have extracted from the newspaper, called "L'Ermite du Mont Liban," published by Mons. Regnault, French Consul at Tripoli, in Syria, and is as follows: