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changes of igneous origin that have already taken place in the earth, and may occasion others not less remarkable, should time -that essential element in the vicissitudes of the globe-be granted, and their energy last.

Sir Charles Lyell, who has shown the power of existing causes with great ingenuity, estimates that on an average twenty eruptions take place annually in different parts of the world; and many must occur or have happened, even on the most extensive and awful scale, among people equally incapable of estimating their effects and of recording them. We should never have known the extent of the fearful eruption which took place in the island of Sumbawa, in 1815, but for the accident of Sir Stamford Raffles having been governor of Java at the time. It began on the 5th of April, and did not entirely cease till July. The ground was shaken through an area of 1000 miles in circumference; the tremors were felt in Java, the Moluccas, a great part of Celebes, Sumatra, and Borneo. The detonations were heard in Sumatra, at the distance of 970 geographical miles in a straight line; and at Ternate, 720 miles in the opposite direction. The most dreadful whirlwinds carried men and cattle into the air; and with the exception of 26 persons, the whole population of the island perished to the amount of 12,000. Ashes were carried 300 miles to Java in such quantities that the darkness during the day was more profound than ever had been witnessed in the most obscure night. The face of the country was changed by the streams of lava, and by the upheaving and sinking of the soil. The town of Tomboro was submerged, and water stood to the depth of 18 feet in places which had been dry land. Ships grounded where they had previously anchored, and others could hardly penetrate the mass of cinders which floated on the surface of the sea for several miles to the depth of two feet. A catastrophe similar to this, though of less magnitude, took place in the island of Bali in 1808, which was not heard of in Europe till years afterwards. The eruption of Coseguina in the Bay of Fonseca, which began on the 19th of January, 1835, and lasted many days, was even more dreadful and extensive in its effects than that of Sumbawa. The ashes during this eruption were carried by the upper current of the atmosphere as far north as Chiassa, which is upwards of 400 leagues to the windward of that volcano. Many volcanoes supposed to be extinct have all

at once burst out with inconceivable violence. Witness Vesuvius, on historical record; and the volcano in the island of St. Vincent in our own days, whose crater was lined with large trees, and which had not been active in the memory of man. Vast tracts are of volcanic origin where volcanoes have ceased to exist for ages. Whence it may be inferred that in some places the subterraneous fires are in the highest state of activity, in some they are inert, and in others they appear to be extinct. Yet there are few countries that are not subject to earthquakes of greater or less intensity; the tremors are propagated like a sonorous undulation to such distances that it is impossible to say in what point they originate. In some recent instances their power must have been tremendous. In South America, so lately as 1822, an area of 100,000 square miles, which is equal in extent to the half of France, was raised several feet above its present level-a most able account of which is given in the Transactions of the Geological Society,' by an esteemed friend of the author's, the late Mrs. Graham, who was present during the whole time of that formidable earthquake, which recurred at short intervals for more than two months, and who possessed talents to appreciate, and had opportunities of observing, its effects under the most favourable circumstances at Valparaiso, and for miles along the coast where it was most intense. A considerable elevation of the land has again taken place along the coast of Chile, in consequence of the violent earthquake which happened on the 20th of February, 1835. In 1819 a ridge of land stretching for 50 miles across the delta of the Indus, 16 feet broad, was raised 10 feet above the plain. The reader is referred to Sir Charles Lyell's excellent 'Principles of Geology,' already mentioned, for most interesting details of the phenomena and extensive effects of volcanoes and earthquakes, too numerous to find a place here. It may however be mentioned that innumerable earthquakes are from time to time shaking the solid crust of the globe, and carrying destruction to distant regions, progressively though slowly accomplishing the great work of change. A most disastrous instance took place on the 15th of December, 1857, in the Neapolitan provinces of La Basilicata and Principato Citeriore, where the destruction was extensive and terrible; the number of victims, according to the official accounts, being

returned at upwards of ten thousand. These terrible engines of ruin, fitful and uncertain as they may seem, must, like all durable phenomena, have a law which may in time be discovered by long-continued and accurate observations.

The shell of volcanic fire that girds the globe at a small depth below our feet has been attributed to different causes. By some it is supposed to originate in an ocean of incandescent matter, still existing in the central abyss of the earth. Some conceive it to be superficial, and due to chemical action, in strata at no very great depth when compared with the size of the globe. The more so as matter on a most extensive scale is passing from old into new combinations, which, if rapidly effected, are capable of producing the most intense heat. According to others, electricity, which is so universally diffused in all its forms throughout the earth, if not the immediate cause of the volcanic phenomena, at least determines the chemical affinities that produce them. It is clear that a subject so involved in mystery must give rise to much speculation, in which every hypothesis is attended with difficulties that observation alone can remove.

But the views of Mr. Babbage and Sir John Herschel on the general cause of volcanic action, and the changes in the equilibrium of the internal heat of the globe, accord more with the laws of mechanics and radiant heat than any that have been proposed. The theory of these distinguished philosophers, formed independently of each other, is equally consistent with observed phenomena, whether the earth be a solid crust encompassing a nucleus of liquid lava, or that there is merely a vast reservoir or stratum of melted matter at a moderate depth below the superficial crust. The author is indebted to the kindness of Sir Charles Lyell for the perusal of a most interesting letter from Sir John Herschel, in which he states his views on the subject.

Supposing that the globe is merely a solid crust, resting upon fluid or semi-fluid matter, whether extending to the centre or not, the transfer of pressure from one part of its surface to another by the degradation of existing continents, and the formation of new ones, would be sufficient to subvert the equilibrium of heat in the interior, and occasion volcanic eruptions. For, since the internal heat of the earth is transmitted outwards by radiation, an accession of new matter on any part of the surface, like an addition of clothing, by keeping it in, would raise the

temperature of the strata below, and in the course of ages would even reduce those at a great depth to a state of fusion. Some of the substances might be converted into gases; and should the accumulation of new matter take place at the bottom of the sea, as is generally the case, this lava would be mixed with water in a state of ignition in consequence of the enormous pressure of the ocean, and of the newly superimposed matter which would prevent it from expanding into steam. Now Sir Charles Lyell has shown, with his usual talent, that the quantity of matter carried down by rivers from the surface of the continents is comparatively trifling, and that the great transfer to the bottom of the ocean is produced at the coast-line by the action of the sea;hence, says Sir John Herschel, "the greatest accumulation of local pressure is in the central area of the deep sea, while the greatest local relief takes place along the abraded coast-lines. Here then should occur the chief volcanic vents." As the crust of the earth is much weaker on the coasts than elsewhere, it is more easily ruptured, and, as Mr. Babbage observes, immense rents might be produced there by its contraction in cooling down after being deprived of a portion of its original thickness. The pressure on the bottom of the ocean would force a column of lava mixed with ignited water and gas to rise through an opening thus formed, and, says Sir John Herschel, "when the column attains such a height that the ignited water can become steam, the joint specific gravity of the column is suddenly diminished, and up comes a jet of mixed steam and lava, till so much has escaped that the matter deposited at the bottom of the ocean takes a fresh bearing, when the evacuation ceases and the crack becomes sealed up."

This theory perfectly accords with the phenomena of nature, since there are very few active volcanoes at a distance from the sea, and the exceptions that do occur are generally near lakes, or they are connected with volcanoes on the maritime coasts. Many break out even in the bottom of the ocean, probably owing to some of the supports of the superficial crust giving way, so that the steam and lava are forced up through the fissures.

Finally, Mr. Babbage observes that, "in consequence of changes continually going on, by the destruction of forests, the filling up of seas, the wearing down of elevated lands, the heat radiated from the earth's surface varies considerably at different

periods. In consequence of this variation, and also in cousequence of the covering up of the bottom of the sea by the detritus of the land, the surfaces of equal temperature within the earth are continually changing their form, and exposing thick beds near the exterior to alterations of temperature. The expansion and contraction of these strata may form rents and veins, produce earthquakes, determine volcanic eruptions, elevate continents, and, possibly, raise mountain chains."

The numerous vents for the internal heat formed by volcanoes, hot springs, and the emission of steam, so frequent in volcanic regions, no doubt maintain the tranquillity of the interior fluid mass, which seems to be perfectly inert unless when put in motion by unequal pressure.

But, to whatever cause the increasing heat of the earth and the subterranean fires may ultimately be referred, it is certain that, except in some local instances, they have no sensible effect on the temperature of its surface. It may therefore be concluded that the heat of the earth, above the zone of uniform temperature, is entirely owing to the sun.

The power of the solar rays depends much upon the manner in which they fall, as we readily perceive from the different climates on our globe. Although the sun is about three millions of miles nearer to the earth in winter than in summer, his rays strike the atmosphere in the northern hemisphere so obliquely that it absorbs a much greater quantity of heat than when they are more direct (N. 217). Indeed it is so great that, when the sun has an altitude of 30°, one half of his heat is absorbed by the atmosphere, and it increases very rapidly as he sinks towards the horizon. However, that heat is not lost it is most beneficial to the earth, being really the heat which supplies the greatest part of that which is radiated into space during the absence of the sun. Professor Dove has shown, by taking at all seasons the mean of the temperatures of points on the earth's surface diametrically opposite to each other, that the average temperature of the whole earth's surface in June, when we are farthest from the sun, considerably exceeds that in December, when we are nearest to him, owing to the excess of water in the southern hemisphere, and that of land in the northern, which gives a general insular climate to the former, and a continental climate to the latter.

The observations of the north polar navigators, and those of

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