life and property. The terremotos, on the contrary, indicate violent horizontal oscillations similar to the wave-movements of the sea, or perpendicular upliftings, as if a power was operating upon the roof of a cavern from the interior, struggling to force it open, and dash it away in fragments with everything upon it. By these last forms of the earthquake, buildings are levelled, cities become heaps of ruins, fissures in the ground are opened, rocks are split, lakes are formed, streams receive a new direction, springs are stopped to gush out in fresh sites, and the general level of considerable regions is sometimes depressed or elevated. Such are the features which enter into these natural convulsions, either in combination, or in a more partial manner.

In

The influence of an earthquake of more than ordinary severity extends to an immense distance from the central seat of action. That which occurred in Chili, in the year 1835, was felt at all places between the parallels of 27° and 40° through thirteen degrees of latitude, approximating to a thousand miles, and through ten degrees of longitude. The most recent formidable earthquake in Europe, that of which Lisbon was the focus in 1755, was experienced at very remote points of the continent. It turned some of the rivers in Switzerland suddenly muddy without any rain, plainly showing a disturbance of their bed, and at Neufchatel the lake swelled to the height of nearly two feet above its usual level. At Portsmouth, a ship in dock and well secured, the Gosport, was pitched backwards and forwards several times by the sudden and violent motion of the water. the moat around Shirburn Castle in Oxfordshire, there was a regular flux and reflux of the water produced. Two miners at work in one of the lead mines at Eyam in Derbyshire, at a depth of more than six hundred feet, noticed the vibration of the earth, which caused some loose pieces of material to drop from the roof and sides of the mine. The lakes of Scotland and Norway, the canals at the Hague, and the springs of Toplitz in Bohemia, gave sensible indications of participating in the catastrophe, which suddenly, after the sun had risen in a serene sky over Portugal, half annihilated the capital, and left signal instances of physical change in that part of the peninsula as monuments of its terrific energy. Extensive, however, as the area through which a severe shock exerts its influence, its more destructive ravages are confined within a comparatively narrow range, which may be called the centre of disturbance, though sometimes there seem to be several foci. Humboldt, in the narrative of his journey to the equinoctial regions of the New Continent, has recorded all that could be learnt respecting the earthquake of the 26th of March 1812, which destroyed the city of Caraccas, with twenty thousand inhabitants of the province of Venezuela. An abridgment of this account, will not only illustrate the human disasters common on such occasions, but the vast area shaken by the subterranean commotions, indicating a common agency exerted at a great depth in the interior of the globe, and bearing with fatal energy upon particular points. Drought was prevalent through the province of Venezuela at the time, and not a drop of rain had fallen for five months around the capital. The day of its destruction broke with a calm air and a cloudless sky, and became excessively hot. It was Holy Thursday, and the population gathered to the churches as usual on the festival. Not any token of danger appeared, till seven minutes after four in the afternoon, when a commotion was felt sufficiently strong to make the bells of the churches ring. The ground continued in a state of undulation, heaving like a fluid under ebullition, till a noise was heard louder and more prolonged than the thunder of the fiercest tropical storm, when the undulations became more violent, and proceeding from opposite directions, and crossing each other, Caraccas was overthrown. Subsidences occurred at the churches of the Trinity and Alta Gracia, and the barracks called El Quartel de San Carlos almost entirely disappeared by the sinking of the ground. The night of Holy Thursday presented a distressing scene of desolation and sorrow, which contrasted sadly with the beautiful aspect which nature

speedily resumed. The thick clouds of dust which rose from the ruins and darkened the air, had fallen to the ground. The shocks had ceased. Never was there a finer or a quieter night. The rounded summits of the Silla mountain were illuminated by the moon, nearly at the full, and the serenity of the heavens seemed to mock the disturbed state of the earth, where under a heap of ruins lay nearly ten thousand of the inhabitants of Caraccas. "In this city," says Humboldt, "was now repeated what had taken place in the province of Quito, after the dreadful earthquake of the 4th of February 1797. Marriages were contracted between persons who for many years had neglected to sanction their union by the sacerdotal blessing. Children found parents in persons who had till then disavowed them; restitution was promised by individuals who had never been accused of theft; and families who had long been at enmity, drew together from the feeling of a common evil." Caraccas was at this period one of the foci of subterranean commotions, which from the beginning of 1811 to 1813 operated on a vast extent of the earth's surface, an area limited by the meridian of the Azores, the valley of the Ohio, and the cordilleras of New Grenada. The shocks fatal to the city were sensibly felt at Honda on the banks of the Magdalena, 620 miles distant. Large masses of earth fell in the mountains, and enormous rocks were detached from the Silla. The lake of Maraycabo underwent considerable diminution, but at Valecillo, the ground opened, and emitted so great a mass of water, that a new torrent was formed, the same phenomenon taking place near Porto Cabello. In all parts the disturbance was more violent in the cordilleras of gneiss and mica-slate, or immediately at their base, than in the plains.

A personal examination was made by Dolomieu and Sir William Hamilton of the surface of the Calabrias after the earthquakes which continued from the beginning of 1783 to the close of 1786, and their survey illustrates the superficial changes produced by the action of such events. Those provinces have been subject to such visitations since the first Greek colonists landed upon their shores, but the most terrible instances in modern times occurred in 1633 and 1783, in the latter of which Sicily largely participated. The soil of the mainland is chiefly composed of modern marine strata, immensely thick, generally of calcareous clay, of a very yielding nature, which was greatly disturbed, and assumed a variety of new forms, under control of the irresistible force acting upon it.

The earth exhibited a variety of motions, called in the Italian accounts vorticoso, orizontale, and oscillatorio, whirling like a vortex, horizontal, or by pulsations or beatings from

the bottom upwards. Valleys underwent extensive and striking alterations through the precipitation into them of masses from the neighbouring hills. Fissures, radiating from a central point, or single horizontal openings of the ground, appeared in various places, as at Polistena, where the ground was rent asunder to

A permanent chasm in the district of Rosiano was a mile long, upwards of a hundred feet broad, and thirty deep. Some of these fissures, after swallowing up houses and men, closed and opened again, so that property was recovered, and the victims of the catastrophe were restored to the rites of burial. The most singular landslip occurred near Seminaria, where a large

olive-ground descended from the heights into the valley, retaining its compactness, a house upon it standing firm, and the olives continued to grow in their new situation, bearing an abundant crop the same year. In the plains, a considerable number of circular funnel-shaped hollows were formed,

filled with water or sand, a consequence of the vorticose or whirling motion of the earth. These are a sample of the effects in one region, of a cause, to the influence of which a vast area of the globe is exposed. We are presented with a continued series of such operations from the earliest times of which we have any knowledge, and hence the fact is sufficiently obvious, that the aggregate has largely altered the physical condition of the globe, though we cannot estimate the amount of the change. The ancients were not much addicted to physical inquiry; they contented themselves with general allusions in their writings to natural phenomena; yet we learn enough from these slight notices to know that, in their day, " Vulcan's habitation" was rife with "terrific flashes," with "noise and terror;"

Vulcani domus

Fulgores terrificos, sonitumque metumque:

and, brief as is the reference of Thucydides to the earthquake at Euboea, writing four centuries before Christ, the leading features of the account will apply to that at Lisbon, or Messina, twenty-one centuries afterwards.

We are left entirely to hypothesis as to the cause of earthquakes and volcanic eruptions, but of the oneness of that cause no doubt remains. The theory first started by Sir Humphrey Davy, suggested by his discovery of the metallic bases of the earths and alkalis, and since elaborated by Dr. Daubeny, has found very general acceptance. According to this hypothesis, which is that of Subterranean Oxidation, the earth is supposed to contain in its interior, at a great depth and in sufficient quantity, the earthy and alkaline metalloids, iron, sulphur, and sulphuretted salts. These are substances which combine with oxygen with avidity, a high temperature and strong inflammation being the result of the combination. A supply of water reaching the interior of the earth, becomes decomposed by contact with the metalloids, and yields its oxygen to them, occasioning the phenomena of combustion and explosion, which may be imitated upon a

FF

small scale, by burying in the ground a moistened mixture of sulphur and iron filings, when the mass becomes gradually heated, takes fire, and explodes. The lava which flows out to the surface in volcanic eruptions, or is driven up in dust and scoriæ, is owing to the violent extrication, through a vent, of the steam which has been generated, accumulated, and confined, the oscillations and heavings of the ground in earthquakes being produced by the action of elastic vapours and gases endeavouring to effect their

Volcano of Orizaba.

escape by a rending of the strata. Such is the hypothesis. It requires the metalloids to exist in the interior of the earth, and the admission of water in sufficient abundance to them, a condition which may be conceded, with strong probabilities in its favour. It has been considered an evidence supporting this theory, that nearly all active volcanoes are situated near the sea, or in ranges of mountains which approach it at certain points, while coast situations have been the foci of the most terrible modern earthquakes, as those of Lima, Lisbon, Messina, and Caraccas. The extinct volcanoes of Auvergne and

the Catecucaumene are indeed apart from the present ocean, and this is true of several sites of former volcanic action; but in those ancient times, when "fire and vapour of smoke" marked the panorama, the relative distribution of land and sea in those localities might be different to what it is at present.

It is difficult to form just views of events occasioning such calamities to the human race as the reduction to instability of the before fixed and firm foundations of the globe. When in a few passing seconds peaceful homes become the sepulchres of their inhabitants, and the roof that long has sheltered them from inclement elements is the engine of their destruction-when scenes verdant through the industry of man are converted into frightful desolations, and cities fall, involving youth, beauty, and innocence in indiscriminate ruin with proficient and inveterate vice-men are prone to reflections questioning the goodness and fitness of things, challenging the arrangements of the Creator in the scheme of the creation. Yet in most cases this is the offspring of a miserable selfishness; for the same parties will gloat over a battle in which their nation has been victorious, though destructive to more than ever perished by any one natural visitation of earthquake, volcano, or pestilence; the human action, at the same time, involving a deep moral guilt which belongs not to the physical phenomena. It may be well to recollect, in relation to these paroxysms of nature, that science and philosophy step in, and suggest relieving considerations. They unfold the long antiquity of the earth, teach us to contemplate it in connection with an era compared to which an age is a span, and unfold the tendency of those milder agencies which are in incessant action upon it, and which, though slow workers, would effect extensive and disastrous changes in the succession of centuries, if there was no counteraction to them. Inequalities of the surface eminently adapt the globe to be the residence of man during his threescore years and ten, and of the myriads of different races of beings that inhabit it. But the waste of the elevated dry land is a gradual yet sure effect produced by the atmospheric and aqueous causes that constantly act upon it. These, without an antagonist power, would, in time, reduce the inequalities of alluvial countries nearly to a uniform level, bring the habitable part of our planet down to the ocean line, and convert scenes of fertility and busy life into vast lagunes and

marshes, which only inferior orders of animals can occupy. The antagonist power is the subterranean upheaving agency-a rare visitant-often at rest for ages-and then counteracting in the twinkling of an eye the effect of the rains, rills, torrents, rivers, atmosphere, and seas, that have been preying upon the soil.

CHAPTER XIII.

THE ATMOSPHERE AND ITS CURRENTS.

T has been mentioned as a part of the planetary constitution of our globe, that a gaseous envelope environs its mass-the atmosphere- which requires the attention of the astronomer, on account of its influence in displacing the celestial bodies, and contributing to their visibility by refracting and reflecting the rays of light. This elastic fluid is the scene of interesting phenomena, and performs important functions in the economy

of nature. Besides being essential to the life of man, and the animal races, whose existence would terminate in a few minutes without the respiration of it;-the exhalation of moisture from the surface of the earth is mainly owing to the common air we breathe, which receives and sustains the vapours formed into clouds, distributes them over different regions by its incessant motions, and tempers by its currents those extremes of heat and cold to which various localities are subject. It is in these last-named offices that the atmosphere demands the notice of the physical geographer. The consideration of its actual constitution does not belong to his province, but a general view of the fluid may be appropriate before we proceed to those agitations and changes which are in constant action, and upon which the welfare of organised beings so materially depends. The atmosphere is, then, an integral portion of the earth, a body of air revolving with the solid mass upon its axis, the higher strata, of course, increasing in velocity with the distance from the axis of revolution. From hence a conclusion may be drawn respecting its height, for an absolute limit is put to its elevation by this feature of its physical condition. There is a point where the centrifugal force, or the tendency to fly off from the centre, will counterbalance the centripetal, or the gravitation towards the centre, and beyond that point the latter will be vanquished. It is obvious that no portion of the atmosphere can extend beyond the point where the two influences balance, or are in equilibrium, and the projectile force becomes greater than that of gravitation, or its projection into space would follow. At the distance of 6.6 radii from the centre of the earth, or at an elevation of 22,200 miles, about the eleventh part the distance of the moon, this point is fixed, beyond which it is impossible for the atmosphere in the smallest quantity to extend. This consideration is only of importance to show that physical laws rigidly restrict it within finite bounds, for any portion of air at that distance must have a tenuity which is utterly inconceivable. The indications of the height of the atmosphere drawn from its weight, as shown by the barometer, reduce its elevation within a vastly circum