torrents of lava from the bofom of its native earth, and its firft crater would be compofed of the fragments of the fame earth. Thus, according to our author, the foundations of the burning mountain would be laid in the bottom of the fea; and even then it would have an hollow cup, or crater on the top, fimilar to that which is to be found on all volcanoes at present. But the question now very naturally occurs, By what means was the internal fire preserved from extinction by the waters of the ocean which were recumbent upon it? To this he replies, that the fire having difpofed the fubftances in fufion to make an eruption, next laid open the earth, and emitted as much matter as it could discharge, with force fufficient to overcome the column of water which would oppose its afcent; but as the ftrength of the fire diminished, the matter discharged was no longer expelled beyond the mouth; but by accumulating there, foon closed up the orifice. Thus only small orifices would be left fufficient for giving vent to the vapours of the volcano, and from which only fmall bubbles of air could afcend to the furface of the water, until new circumftances, fuch as original. ly gave occafion to the eruption, again took place in the bowels of the earth, and produced new eruptions, either through the fame or other mouths. The appearance of the fea over the new-formed volcano, in its ftate of tranquility, would then be fimilar to what it is betwixt the islands of Bafilizzo and Pariaria: columns of air-bubbles are there afcending at the depth of more than thirty feet, and burst on their arriving at the furface. This air would continue to difengage itself with little difturbance, as long as it iflues forth only in fmall quantity, until, at the very inftant of explosion, when prodigious quantities, generated in the burning focus, would make their way all at once, and the fame phenomena which originally took place, would again make their appearance.'

""

A volcano, while under water, cannot act precisely as it does in the open air: its eruptions, though equally strong, cannot extend to fo great a diftance: the lava accumulates in greater quantity round the crater; the fands, afhes, and pozzolano are not carried away by the winds, but are depofited around its edges, and prevent the marine fubftances which are driven that, way by the waters from entering; thus they agglomerate with thefe bodies, and thus a pyramidal mount is forined of all the materials together.

In this manner Mr. Houel fuppofes, that the mountain was gradually raised out of the fea, by the accumulation of lava, &c. at every eruption, and that the cavern of the volcano was

gradually

gradually enlarged, being worked down further into the earth by the continued action of the ftones which the volcano is perpetually throwing up and receiving back again; and that, at laft, the matter being in a proper ftate of fufion, it was thrown out at the top of the mountain to accumulate on its fides.-.Mr. Houel's opinion about the volcanic fire, is as follows--"We cannot form any idea of fire fubfifting alone, without any pabulum, and unconnected with any other principle. We never behold it but in conjunction with fome other body, which nourishes it, and is confumed by it. The matter in fufion, which iffues from the focus, is but the incombuftible part of that which nourishes the fire, and into the bofom of which that active principle penetrates in fearch of pabulum. But as the fire acts only in proportion to the facility with which it can diffolve and evaporate, I am of opinion that it is only the bottom of the volcano on which it acts; and that its action extends no farther than to keep these substances which it has melted in a conftant state of ebullition; that fufible matter being discharged from the mouth of the volcano, and hardening as it is gradually cooled by the action of the air, produces that species of ftones which are diftinguifhed by the name of lavas. This lava, even when in the focus, and in a state of fluidity, muft alfo poffefs a certain degree of folidity, on account of the gravity and denfity of its particles; it therefore oppofes the fire with a degree of refiftance which irritates it, and requires, to put it into a state of ebullition, a power proportioned to the bulk of the mass."

"That quantity of matter, when diffolved by the action of the fire, muft conftantly resemble any other thick fubstance in the fame ftate. Small explotions are produced in various parts over the furface of every fuch fubftance while in a state of ebullition; and, by the bursting of these bubbles, a great number of fmall particles are fcattered around. This is the very procefs carried on in the focus of a volcano, though on a fcale immensely more large; and the vaft explosions there produced expel every body which lies in their way with the utmoft violence; nor is there any piece of lava which falls down from the upper part of the arch of weight fufficient to resist this violent centrifugal force."

No estimate can be made of the power of thefe explosions, but by obferving the obftacles they overcome, and what enormous bodies are raised up and thrown to an immenfe height and distance. Such vaft pieces of lava are to be feen on the top of Vefuvius and Lipari, that the projectile force by which they

B 2

have

have been thrown out, appears altogether incredible. No perfon can harbour the leaft fufpicion of their having been laid there by any human power; and the appearance of them demonftrates that they have been ejected from the bottom of the volcano, not in a state of fufion, but coherent and folid. A piece of lava lies on the top of Ætna of more than a cubic fathom in bulk, and whose weight, therefore, cannot be lefs than fixteen tons. What an amazing force, then, must it have required, not only to raise this enormous mass from the volcanic focus, but to make it describe a parabola of about a league in diameter, after it had come out of the crater!"

"When we confider how much the volcanic focus is funk below the base of the mountain, that the mountain itself is 10,000 feet high, and that confequently there must have been a power fufficient to raise fuch a mass 12,000 feet perpendicular, the boldeft imagination must be loft in amazement. This may serve to give us fome idea of the nature of that power which operates in the foci of volcanoes; a power which is unknown and inconceivable, and may justly be reckoned among the mysteries of nature.

The pabulum by which the internal fire is supported, Mr. Houel thinks to be substances contained in the mountain itself, together with bitumen, fulphur, and other inflammable materi als, which may from time to time flow into the focus of the volcano in a melted ftate, through fubterraneous ducts; and the explosions he ascribes to water making its way in the fame manner the water is converted into fteam, which fills the cavern, and pushes the melted lava out at the crater. This opinion is corroborated by the copious smoke which always precedes an eruption. But, combined with the water, there is always a quantity of other fubftances whofe effects precede, accompany, or follow the eruptions, and produce all the various phenomena which they display. The eruption of water from Ætna in the year 1775, proceeded undoubtedly from this caufe. The fea, or fome of the refervoirs in Ætna or the adjacent mountains, by fome means discharged a vast quantity of water into the focus of the volcano: the water was inftantly refolved into vapour, which inftantly filled the whole cavern, and iffued from the mouth of the crater. As foon as it made its way. into the open atmosphere, it was condensed again into water, which ftreamed down the fides of the mountain in a dreadful and deftructive torrent.

Thus we have given a view of Mr. Houel's theory; according to which volcanoes originally began at the bottom of the

fea; and not only the mountain, but all the adjoining country, was formed by fucceffive eruptions: it is rather a theory of mountains raised by fubterraneous heat, than of volcanoes, and does not attempt to explain the origin of the fire, which is the principal difficulty; neither does his theory account for the immense height to which matters are fometimes thrown during eruptions. This, indeed, it is impoffible to account for, without fuppofing that the refiftance of the air is diminished. It is well known that the air forms a refiftance to all bodies moving in it, according to the velocity with which they move--What ought that refiftance to be, then, on irregular masses of rock, or ftreams of liquid lava? Nevertheless, in the great eruption of Vesuvius in 1779, Sir William Hamilton informs us, that a vast stream of lava was projected to the height of at leaft 10,000 feet above the top of the mountain. Had the air refifted this liquid matter as it does a cannon-ball, it must have been dashed in pieces almost as foon as it iffued from the crater. Either the extreme heat of the lava, therefore, or fome other cause, must have contributed very much to diminish, or rather, in a manner, to annihilate the refiftance of the atmofphere at that time. As for the lighter materials, though they may be fuppofed to be carried to a vaft diftance by the wind, after being projected to a great height in the air, it is inconceivable how their motion was not fuddenly stopped, and they fcattered all around the top of the volcano by the resistance of the blaft. Substances of this kind, when quietly carried up with smoke, will indeed fly to a great distance; for we are af fured that the afhes of the great fire at London, in 1666, were carried by the wind to the diftance of fixteen miles. It is, therefore, the lefs incredible that thofe of the great eruptions of Vefuvius, in 1779, fhould be carried to the diftance of 100 miles, as we are informed was the cafe.

To account for the volcanic fire, Dr. Woodward and others have had recourse to the hypothefis of a central fire, to which the volcanoes are fo many chimnies or fpiracles. Dr. Hutton, in his Theory of the Earth, adopts the fame opinion; but as it did not immediately concern the fubject of which he treated, he evades any queftion concerning its origin, by declaring himself fatisfied of its exiftence, without any inquiry into its origin.

Others, as Dr. Lifter, have had recourse to the well known experiment of the fermentation of fulphur and iron, which will take fire when mixed in confiderable quantity and moistened with water. Pyrites, therefore, which are a natural

mixture

mixture of these two substances, it is supposed, may naturally give rife to volcanoes. Inftances are indeed adduced which undeniably prove that thefe fubftances will fpontaneously take fire when thrown together in large heaps. Of this we have a remarkable example in the following anecdote--

cr

"A covetous copperas maker at Deptford, having bought up all the pyrites he could find, in order to ruin the trade of his neighbours, collected a vaft quantity below a fhade, in order to fecure them from the rain. He was foon, however, punished for his avarice; for the pyrites began to fmoke, glowed like red-hot coals, and melted into a kind of vitrified and partly metallic fubftance, grievoufly annoying the neighbourhood for a long time with the fulphureous fteam they emitted." Beds of pyrites, therefore, taking fire in the earth by means of a fermentation occafioned by water, are now generally fuppofed to be the cause of volcanoes; and that volcanoes are generally near the fea, is thought to confirm this hypothesis.

When the matter is properly confidered, however, it must be evident that neither of thefe hypothefes can answer the purpofe. The central fire of Dr. Woodward and others is a caufe too magnificent even for volcanoes: if any fuch fire is fuppofed, we muft imagine a burning globe in the centre of the earth, whofe heat is fufficient to vitrify the most folid and refractory terreftrial fubftances. But of what dimenfions are we to fuppofe this globe? Is it one, two, three, four, or more thoufand miles in diameter? Very large indeed it must be; for we can scarce fuppofe that ftones could be projected even from the depth of 500 miles, into the air. But even this fuppofition is inadmiffible; for as the fire of volcanoes is at times exceedingly augmented, from fome caufe or other, were the cause general, as it must be in cafe of a burning central globe, the whole number of volcanoes exifting on the earth would be in a state of eruption at once. Befides, if we were to fuppofe a burning globe of 7000 miles in diameter to fuffer the least dilatation throughout its vaft bulk, which must be the undoubted confequence of an augmentation of heat from an unknown cause, all the volcanoes in the world would not be fufficient to give vent to it, though they should spout forth inceffant cataracts of lava for centuries together. A diffolution of the whole globe muit therefore undoubtedly take place: and though we should leffen the diameter of our burning globe by 1000 miles, our difficulties would be as far from being removed as before. Volcanic fire, therefore, cannot originate from any general collection of burning materials difperfed throughout the vast mass of