swells up to a great height; its waters seem to be entirely separated, and from the place of separation currents of air smoke aud flame are discharged. Similar effects have been observed on lakes, ponds and rivers.

The duration of a shock rarely exceeds a minute, and perhaps very few have continued nearly so long: but the shocks often follow each other in rapid succession, and from the dread and terror excited in the minds of persons exposed to their ravages, their duration nay appear far to exceed the truth.

History, antient and modern, is full of the disasters occasioned by earthquakes. In June 1690, the town of Port Royal, at the entrance of Kingston harbour, in Jamaica, was almost entirely overthrown by an earthquake, and swallowed up in the sea. On the 1st of November 1755, came on at Lisbon, a concussion of the earth, which for its extent was one of the most extraordinary on record: in the course of six minutes the greater part of that great capital was levelled with the ground, and thousands of the inhabitants buried under the ruins: the bed of the river Tagus in some places heaved up to the surface of the water: the bar at the entrance of the river was for a time dry from shore to shore; in another part the water rose instantaneously to the height of fifty feet.

A very singular circumstance attending this earthquake was the prodigious extent in which its effects were more or less sensible: all over Portugal and Spain shocks were felt, even at Madeira the earth was sensibly agitated, and the sea rose suddenly fifteen feet above its usual height. In Corsica, at Milan and Turin in Italy, the earthquake was evidently observed. In Britain, Norway, and Sweden, the rivers and lakes were powerfully agitated. Ships out in the Atlantic ocean felt a sudden commotion in the waters, as if they had struck upon a rock.

One of the most calamitous earthquakes of which we have

any

any accurate account, was that which visited Calabria, in the south of Italy, in 1783. Of this event it has been observed, that if round the town of Oppido as a centre, with a radius of twenty-two miles, a circle were. described on a map of Italy, it would comprehend all the towns, villages, castles, &c. which were utterly destroyed, with the spots where the greatest mortality prevailed, and where happened the most sensible alterations on the surface; again, a circle described; round the same centre, with a radius of seventy-two miles, would inclose the whole country in any way visibly affected by that earthquake.

Britain has of late years shewn symptoms of earthquakes, feeble indeed and unimportant, but unequivocal and alarming. In a tract surrounding the village of Comrie, in the county of Perth, in Scotland, at different periods, noises and slight concussions have been perceived; and in the three last months of 1790, these were so considerable as to attract the general attention of the country, not only the earth but the lakes and rivers being very sensibly agitated.

OF VOLCANOES.

Burning mountains or volcanoes are found in almost every part of the globe. Hecla in Iceland, and a volcanoe in Tierra del Fuego, at the extremity of South America, occupy the limits of the known world. The numbers of volcanoes at present known is about a hundred: those of Europe are well known, Hecla already mentioned, Vesuvius in the vicinity of Naples, and Etna in Sicily; to these must be added the volcanoes in the Lipari islands on the north of Sicily.

Volcanoes always appear in the summits of mountains, none having yet been found in the midst of plains; the existence of volcanoes in the bottom of the ocean is no

exception, for these are also in the highest points of hills under water.

Volcanic eruptions have been ascribed to the action of the waters of the sea breaking in upon an immense quantity of melted or burning matter, to that of fires in the bowels of the earth, and to the decomposition of different substances, by which great quantities of inflammation and heat are produced.

The first symptom of an eruption is the increase of the smoke in fair weather; this is of a whitish colour, but after some time black smoke is sent up in the midst of the white column. These appearances are usually accompanied by explosions; and the black smoke is followed by a reddish flame. Showers of inflamed stones are next thrown up of a great size and to a prodigious height. During the eruption of Vesuvius, in the beginning of 1792, inflamed masses of rock were projected to a height of two thousand feet above the mountain top, and were distinctly visible in the night from a distance of nine miles. Along with the stones clouds of ashes are also emitted and carried by the winds to very great distances. These appearances, which daily inerease in frequency and violence, are generally preceded and accompanied by earthquakes and hollow noises from the bowels of the earth.

The smoke rising from the mouth or crater of the volcano (so called because it resembles a cup or funnel) is observed to be highly electrical; flashes of lightning dart in zig-zage through the column, often attended by thunder; at last the lava begins to flow. The lava is a current of melted matters, sometimes boiling over at the top, and often when the mountain is very high, as at Mount Etna, breaking out at the side and flowing down to the bottom. At its first discharge lava is in a state of most intense ignition, greatly superior to any thing that can be produced in our diminu tive furnaces. The lava proceeding from Vesuvius had the appearance

appearance of a river of red-hot and liquid metal, such as is seen in a glass-house, on which floated large masses of cinder half-lighted, rolling over one another down the mountain side, and forming a most magnificent cascade. When we consider the materials of which lava consists, which are the common substances found in the earth, namely, stones, metallic ores, clay, &c. we may form some notion of the intense heat within a volcanoe; for such ma terials cannot, by the most powerful furnaces, be brought to fusion without the addition of very fusible salts, such as alcali, nitre, &c.

Currents or rivers of lava on the slopes of Etna have been measured from fifteen to twenty miles in length, six or seven miles in breadth, and fifty feet in depth: the lavas from Vesuvius, a mountain far less elevated, extend six or seven miles from the summit, some of them above a mile in breadth, on a general depth of sixty or seventy feet.

The lavas of Etna, and Vesuvius take a fine polish, and are frequently manufactured into boxes, tables, &c.: in the British Museum are several capital specimens of lava, in both its natural and its polished state.

The streets of Naples are in general paved with polygonal pieces of lava, perhaps in imitation of the magnificent road opened from Rome to Capua, by the CensorAppius Claudius, about 312 years before the Christian æra, in many parts of which the antient pavement of lava is observable, still very entire, consisting of pieces of five, six, or seven sides, and from twelve to twenty-four inches in diameter. The lately uncovered town of Pompeii is supposed to have been overwhelmed by ashes from Vesuvius, about the year 79 of the Christian æra, during an irruption in which Pliny the elder perished, from his eagerness to observe and understand the phænomena, and which is so characteristically described to us in an epistle from his nephew to Tacitus the historian. At one of the gates of this most interesting town are seen

deep

deep ruts worn by carriage wheels, in a pavement of very compact lava. So little was the nature of the pavement of the Appian way known, in the latter times of the Roman empire, that Procopius, who flourished under Justinian about A. D. 560, conjectures that the stones were drawn from some quarter very remote from Italy, because no quarries of a similar substance were then known in that country.

To form a proper theory of the earth, it is necessary to trace the series of those revolutions which have occurred ou its surface, to explain their causes, and so to connect together all the indications of change that are found in the mineral kingdom. The formation of such a theory, therefore, requires an accurate and extensive examination of the pheenomena of geology, and it supposes natural knowledge to have attained a very high degree of improvement. In science there is perhaps no research more arduous than this, none where the subject is so complex, where the appearances of objects are so greatly diversified, or so widely distributed, where the operating causes are so remote from the sphere of ordinary observation.

With such requisites and under such difficulties, it is not surprising that so many ingenious men who have attempted to construct theories of the earth should have failed: to

form such a theory requires a prodigious accumulation of facts, together with a talent for observation and arrange ment, seldom united in any one individual. We are not, however, to suppose that a correct theory is inattainable, although some may think it an arrogant and presumptuous undertaking to aim at explaining how the present state of the globe, and the revolutions it has undergone, have been produced.