during the consulate of Lucius Marcus and Sextus Julius, in the Roman province of Mutina. He relates, that two mountains felt so tremendous a shock, that they seemed to approach and retire with a most dreadful noise. They at the same time, and in the middle of the day, cast forth fire and smoke, to the dismay of the astonished spectators. By this shock, several towns were destroyed, and all the animals in their vicinity killed. During the reign of Trajan, the city of Antioch was, together with a great part of the adjacent country, destroyed by an earthquake; and about three hundred years after, during the reign of Justinian, it was again destroyed, with the loss of 40,000 of its inhabitants. Lastly, after an interval of sixty years, that ill-fated city was a third time overwhelmed, with a loss of 60,000 souls. The earthquake which happened at Rhodes, upwards of 200 years before the Christian era, threw down the famous Colossus, together with the arsenal, and a great part of the walls of the city. In the year 1182, the greater part of the cities of Syria and of the kingdom of Jerusalem were destroyed by a similar catastrophe; and in 1594 the Italian writers describe an earthquake at Puteoli, which occasioned the sea to retire 200 yards from its former bed. The great earthquake of 1755 extended over a tract of at least 4,000,000 of square miles. It appears to have originated beneath the Atlantic Ocean, the waves of which received almost as violent a concussion as the land. Its effects were even extended to the waters in many places where the shocks were not perceptible. It pervaded the greater portions of the continents of Europe, Africa, and America; but its extreme violence was exercised on the south-western parts of the former. Lisbon, the Portuguese capital, had already suffered greatly from an earthquake in 1531; and, since the calamity about to be described, has had three such visitations, in 1761, 1765, and 1772, which were not however attended by equally disastrous consequences. In the present instance, it had been remarked, that, since the commencement of the year 1750, less rain had fallen than had been known in the memory of the oldest of their inhabitants, unless during the spring preceding the calamitous event. The summer had been unusually cool, and the weather fine and clear for the last forty days. At length, on the 1st of November, about forty minutes past nine in the morning, a most violent shock of an earthquake was felt; its duration did not exceed six seconds; but so powerful was the concussion, that it overthrew every church and convent in the city, together with the royal palace and the magnificent opera-house adjoining to it; in short, not any building of consequence escaped. About onefourth of the dwelling-houses were thrown down; and, at a moderate computation, 30,000 individuals perished. Between the 1st and 8th of November, twenty-two shocks were reckoned. This earthquake was also felt at Oporto, Cadiz, and other parts of Europe, and equally severe in Africa. A great part of the city of Algiers was destroyed. In many places of Germany the effects of this earthquake were very perceptible; but in Holland the agitations were still more remarkable. The agitation of the waters was also perceived in various parts of Great Britain and Ireland. At sea, the shocks of this earthquake were felt most violently. Among other catastrophes, the captain of the Nancy frigate, off St. Lucar, felt his ship so violently shaken, that he thought she had struck the ground, but on heaving the lead, found she was in a great depth of water. The earthquakes in Sicily and the two Calabrias began on the 5th of February 1783, and continued until the latter end of the

May following; doing infinite damage, and exhibiting at Messina, in the parts of Sicily nearest to the continent, and in the two Calabrias, a variety of phenomena. The earth was in a constant tremor, and its motions were various, being either vertical or whirling round,-horizontal or oscillatory, that is, by pulsations or beatings from the bottom upwards. There were many openings, or cracks in the earth; and several hills had been lowered, while others were quite level. In the plains, the chasms were so deep that many roads were rendered impassable. Huge mountains were severed, and portions of them driven into the valleys, which were thus filled up. The total amount of the mortality occasioned by these earthquakes in Sicily and the two Calabrias, was, agreeably to the official returns, 32,367; but Sir William Hamilton thought it still greater, and carries his estimation to 40,000, including foreigners. The shocks felt since the commencement of these formidable earthquakes amounted to several hundreds; and among the most violent may be reckoned the one which happened on the 28th of March. It affected most of the higher parts of Upper Calabria, and the inferior part of Lower Calabria, being equally tremendous with the first. Indeed these shocks were the only ones sensibly felt in the capital, Naples. With relation to the former, two singular phenomena are recorded. At a distance of about three miles from the ruined city of Oppido, in Upper Calabria, was a hill, having a sandy and clayey soil, nearly 400 feet in height, and nearly 900 feet in circumference at its base. This hill is said to have been carried to the distance of about four miles from the spot where it stood, into a plain called Campo de Bassano. At the same time, the hill on which the city of Oppido stood, and which extended about three miles, divided into two parts, being situated between two rivers, its ruins filled up the valley, and stopped their course, forming two large lakes, which augmented daily. By the earthquake experienced in Chili in 1822, a great line of coast is stated to have been lifted permanently up to the height of several feet above its former level; and it deserves remark, that though earthquakes are sometimes felt in the interior of countries, their most terrible effects occur chiefly along the coast. On the 2d March, 1825, the city of Algiers was visited with a tremendous earthquake, which destroyed at least 10,000 human beings. It is worthy of remark, that the same phenomena which generally precedes the eruption of Etna and Vesuvius, occurred at Bluda, on this occasion; namely, all the wells and fountains in the neighbourhood became perfectly dry. The barometer had fallen gradually for some days before the earthquake; and the thermometer rose suddenly from 58 to 62 degrees on the day it happened.

CHAP. II.-OF THE SEA.

THE sea is one of the most important objects of Physical Geography; and seems not less necessary to the existence of man himself, than the solid earth upon which he treads. It absorbs and decomposes the noxious particles of the atmosphere; and if it were dried up, the earth would become as arid and unfruitful as a desert. Its various basins-which, with the exception of the Caspian, all stand in connexion with each otherfacilitate the transactions of commerce, and the intercourse of nations; and its productions form a valuable branch of industry in every maritime country.

Bed, Depth, and Level.] The bed or basin of the ocean, being only a continuation of the land, exhibits the same inequalities of surface which continents present. Were the sea dried up, it would present a scene of mountains, valleys, rocks, and plains, covered in some instances with their own peculiar vegetation, and the abode of various species of animals. The depth of the sea varies greatly in different places. The greatest depth ever measured was that ascertained by Mr. Scoresby, the captain of a Greenland whaler, who sunk a very heavy lead in the Greenland Sea, to the depth of nearly 4,700 feet, without finding ground. Our instruments are too weak or imperfect to ascertain greater depths; but it seems probable that in no place does the sea exceed 30,000 feet in depth. La Place, in his Mechanique Celeste, has calculated the mean depth of the sea to be 4 leagues from those oscillations of the ocean which involve in their expression the rotation of the earth, and which are affected by the depth of the saline fluid. Where the coast is bold and rocky, we sometimes find an unfathomable sea close to the steepest rocks; but where the shores decline gradually towards the sea, the depth of the sea usually decreases as we approach them. According to the laws of gravitation, by which in all connected bodies of water, the higher parts must flow towards the lower, till they attain the same level, the level of the ocean is, generally speaking, the same everywhere. The only exception to this position may perhaps be found in gulfs and inland seas, which have only a slight communication with the ocean. In general, small portions of sea, open only to the east, have a higher level on account of the accumulation of the waters being driven into these gulfs, as into an alley without an outlet, by that general movement of the sea from E. to W., of which we shall hereafter speak. "There is, perhaps," says Arnot, in his valuable Elements of Physics, “nothing which illustrates in a more striking manner the exact accordance of nature's phenomena with the few general expressions or laws which describe them all, than the perfect level of the ocean as a liquid surface. The sea never rises or falls in any place, even one inch, but in obedience to fixed laws, and therefore changes may generally be foreseen and allowed for. For instance, the eastern trade-winds and other causes force the water of the ocean towards the African coast, so as to keep the Red Sea about twenty feet above the general ocean-level; and the Mediterranean Sea is a little below that level, because the evaporation from it is greater than the supply of its rivers, causing it to receive an additional supply from the Strait of Gibraltar; but in all such cases the effect is as constant as the disturbing cause, and therefore can be calculated upon with confidence. Were it not for this perfect exactness, in what a precarious state would the inhabitants exist on the sea-shores, and the banks of low rivers. Few of the inhabitants of London, perhaps, reflect, when standing close by the side of their noble river, and gazing on the rapid flood-tide pouring inland through the bridges, that although sixty miles from the sea, they are placed as low as persons sailing upon its face, where perhaps at the time there may be tossing waves, covered with wrecks and the drowning. The horrible destruction that would follow any alteration in the level of the ocean may be judged of by the effects of occasional floods, produced by rains and melting snow in the interior of

In the second volume of the Transactions of the Wernerian Natural History Society, we observe a very interesting map of the bed of the German Ocean by Mr. Stevenson, This is the only map of the kind we have yet seen.

countries, or by these combined with winds and high tides on the coast. The flood at St. Petersburg, in 1825, was dreadful, in which strong westerly winds had retarded the flow of the Neva so much that the water rose forty feet (the height of an ordinary house) above its usual mark, covered all the low parts of the town, and destroyed thousands of human beings. In Holland, which is a low flat, formed chiefly by the mud and sand brought down by the Rhine and neighbouring rivers, much of the country is below the level of the common spring-tides, and is only protected from daily inundations by artificial dikes or ramparts of great strength. What awful uncertainty would hang over the existence of the Dutch if the level of the sea were subject to change: for while we know the water of the ocean to be seventeen miles higher at the equator than at the poles, owing to the centrifugal force of the earth's rotation; were the level, as now established, from any cause, to be suddenly changed but ten feet, millions of human beings would be the victims. Where inundation is regularly periodical, as in the Nile, the hurtful effects can be guarded against, and it may even become useful, by fertilizing the soil. Tracts of land in contact with rivers, and of an elevation between the levels of ebb and flood-tide, may be kept constantly covered with water, by surrounding them with dikes, and opening the sluices at high water only; or they may be kept constantly drained, by opening the sluices only at low water. A vast extent of rice-fields near the mouths of rivers in India and China are managed in this way, the admission or exclusion of water being regulated by the age of the rice-plant.

Colour.] The colour of the ocean is generally of a deep bluish green, particularly in the deeper seas; as the depth diminishes towards the coasts, the water assumes a lighter shade. This apparent colour of the sea may be explained upon the same principle as that of the azure blue of the atmosphere. Both fluids are colourless when in a glass; the air reflects chiefly the most refrangible rays of light, viz. the violet, indigo, and blue, and therefore usually appears of an azure colour, the result of a mixture of these: but the sea, from its density and depth, is able to reflect not only many blue and violet, but also some of the less refrangible rays in sufficient proportion to compose a greenish blue. The other shades in the colour of sea-water depend on illusory or local causes. The green and yellow shades of the sea arise from marine plants; a distinct shade is often communicated to its surface by the presence of myriads of minute insects: and in shallow water, the light reflected from the sand at the bottom often gives a reddish hue to the surface.5 In the West Indies, where

"The floor is of sand like the mountain-drift;
And the pearl-shells spangle the flinty snow,"

the waters of the ocean are often so beautifully transparent, as to exhibit the minutest object they contain or cover at the depth of several fathoms. In the Gulf of Guinea the sea is white; and around the Maldive islands it is black.

• Experiments were made during the voyage of the Coquille, to ascertain at what depth in the sea an apparatus became invisible, composed of a plank two feet in diameter, painted white, and weighted, so that on descending it should always remain horizontal. The results varied much. At Offale, in the Island of Waigou, on the 13th of September, the disc disappeared at the depth of 59 feet-the weather calm and cloudy; on the 14th the sky being clear, it disappeared at the depth of 75.3 feet. At Port

Phosphorescence.] A very curious and magnificent spectacle is often presented at night by the luminous appearance of the sea, a phenomenon which seamen generally regard as the precursor of blowing weather. It is of most frequent occurrence in summer and autumn. Forster distinguishes three species of marine phosphorescence. The first is generally seen close to a ship when sailing before a fresh wind, and forms a tail of light in the wake of the ship; at other times, during stormy weather, it spreads over the whole surface of the sea, clothing it apparently in a sheet of fire. This species he ascribes to electricity. The second kind of marine phosphorescence, penetrates beneath the surface; and when a quantity of the illuminated water is put into a vessel, it retains the brilliance as long as it is kept agitated, but loses it as soon as the agitation subsides. This species occurs during dead calms or in very hot weather, and seems to be a true phosphoric light, emanating from partieles of putrid animal matter suspended in the water. The third species exceeds the two former in intensity of brilliance; and Forster having attentively examined some of the shining water, expresses his conviction that the appearance is occasioned by innumerable minute animals of a round shape, moving rapidly through the water in all directions, like so many luminous sparks. He imagines that these small gelatinous specks may be the young fry of certain species of some medusæ or blubber. M. Dagilet and M. Rigaud observed several times, and in different parts of the ocean, such luminous appearances attended by vast masses of different animalculæ ; and a few days after, the sea was covered near the coasts, with whole banks of small fish in innumerable multitudes, which they supposed had proceeded from the shining animalculæ. But M. le Roi, after giving much attention to this phenomenon, concludes, that it is not occasioned by any shining insects, especially, as, after carefully examining with a microscope some of the luminous points, he found them to have no appearance of an animal; and he also found, that the mixture of a little spirit of wine with water just drawn from the sea, would give the appearance of a great number of little sparks, which would continue visible longer than those in the ocean: the same effect was produced by all the acids, and various other liquids.

Taste and Weight.] When the water of the ocean is subjected to chemical analysis, it appears to be a mixture of fresh water with muriatic acid, sulphuric acid, fixed mineral alkali, magnesia, and sulphated lime. These ingredients are found in sea-water all over the globe, and very nearly in the same proportion to each other: differing only in the total amount of their saline contents. The taste of the ocean is salt and disagreeably bitter; but there is not such a large proportion of common salt present in it as in the water of brine-pits and salt-springs. The quantity of saline ingredients in the waters of the ocean varies from 1-10th to 1-50th part. Mr. Kirwan makes the average quantity about 1-28th. The quantity, however, varies, even in the same latitude, during the rainy and dry seasons, and according to the distance from land and the mouths of great rivers. Near Walloë in Norway, where there is a salt-pit, it has been remarked that the sea-water taken at the surface contains 1-24th of its weight of saline matter at the moment the

Jackson, on the 12th and 13th of February it was not visible at more than 38.3 feet in a dead calm; the mean at New Zealand, in April, was 3.28 feet less; at the isle of Ascension, in January, under favourable circumstances, the extreme limits in eleven experiments were 28 and 38 feet.