and irregularly beneath its waters. All that has been learned from the soundings of navigators establishes the fact, that there are shallow shoals and banks, deeper flats and plateaux, and still deeper troughs and valleys; and that were the whole laid dry, we should have presented to us much the same kind of inequalities as are presented by the surface of the land. Indeed, when situated on a lofty mountain and looking down on the hills and valleys below, we see before us much the same irregularity of surface as that which must lie beneath the waters of the ocean. The sea-bed is but the submerged surface of former lands, and, unless perhaps in the instance of coral-reefs and submarine volcanoes, there is no foundation for the belief that the troughs and precipices of the ocean are sharper and more abrupt than those of the dry land. If the inequalities of the land are worn and rounded by meteoric agency, and masked by a covering of soil, so the inequalities of the ocean-bed were worn down before submergence, and have since been masked, except in the courses of tides and currents, by a still deeper covering of sediment and drifted debris.

117. As a general rule, it may be stated that, where the land slopes gradually towards the ocean, the waters also deepen gradually; and, on the other hand, where the land descends abruptly, the sea deepens, in like manner, suddenly and abruptly. In fact, this is only the natural consequence of slope and counter-slope, and a great relation to which there is scarcely an exception. The northern plains of Russia and Siberia, for example, slope gradually into the shallow bed of the Arctic Sea, just as the abrupt terminations of South America, Africa, and Australia dip suddenly into the deeper waters of the Southern Ocean. The level plains of China spread gently outwards into the shallow waters of the Yellow Sea; the low shores of eastern England, in like manner, slope slowly into the comparatively shallow basin of the North Sea; while, on the other hand, the precipitous coasts of Norway dip suddenly down into a corresponding depth of water. This fact, that low lands are generally bordered by shallow seas and high lands by deeper water, affords no idea, however, of the depths of distant and central expanses, and for these we must either appeal to theoretical deduction or to actual observation.

118. So far as experiment is concerned, comparatively little is known of the absolute depth of the ocean; and even where deep soundings have been made, there has been great liability to error, partly from the imperfection of the apparatus employed, and partly from the chance of the line being deflected from the perpendicular by the force of under-currents. The common notion that the extreme depths of the sea correspond to the extreme

heights of the land—that is, as the highest mountains rise little above five miles, so the greatest depths sink little below that amount has no foundation in fact, there being no necessary connection between the two phenomena. The mean elevation of all the land-continents and islands, mountains and plains-has been estimated by Humboldt at somewhat less than 1000 feet; and the mean depth of the ocean has been calculated by Laplace, from tidal waves and kindred phenomena, to be at least 21,000 feet, or about four English miles. We know, however, that a very large proportion of the ocean is comparatively shallow, and not a tithe of this depth; and therefore, to make up the mean, some other portions must be proportionally deeper, and to the extent, it may be, of eight or ten miles. Indeed, soundings (no doubt open to question) have been made in the South Atlantic, both by British and American navigators, varying from 27,000 to 46,000 feet; and soundings perfectly reliable have been taken in the North Atlantic, off the bank of Newfoundland, to the depth of 25,000 feet; while from calculations on the velocity of tidal waves, which are found to proceed according to the depth of the channel, it has been estimated that the extreme depths of the same ocean are about 50,000 feet, or more than nine miles.

119. Regarding the water in the ocean as a constant quantity, it must follow that the shallower the sea-bed the more expansive its area; and vice versa, the deeper the trough the more circumscribed the area of the waters. Altogether, and according to our present information, we may admit a mean depth of four miles for the ocean, and reliable soundings to the extent of five miles; but believe, partly on experiment, and partly on theoretical grounds, that several portions sink to the depth of eight or ten miles. It may also be stated in general terms that the Atlantic, averaging from three to five miles, is, on the whole, deeper than the Pacific, and yet the great "Telegraphic Plateau" stretching from Cape Clear to Cape Race, a distance of 1640 miles, is only about 11,000 or 12,000 feet in depth; that great depths (from four to six miles) have been determined in the Indian and Southern Oceans; that the Antarctic becomes shallower as we approach the pole; and that the Arctic, of moderate depth, is characterised by great irregularity and diversity. With regard to the minor seas, the greatest ascertained depth in the Mediterranean is about 13,000 feet; in the Red Sea, 6300 feet; in the Baltic, 840 feet; in the Caribbean Sea, 14,000 feet; and in the Gulf of Mexico, about 8000 feet.

Temperature, Colour, Luminosity, &c.

120. Respecting the temperature of the ocean, few reliable or sufficiently extended observations have yet been made, either as regards its various areas or its successive depths. We know, however, that it is more equable than that of the land, and that, though the superficial portions are colder in summer than the surrounding atmosphere of any contiguous terrestrial district, they are in winter always several degrees higher-thus exercising the function of a great storehouse of heat for modifying and equalising the climates of the adjacent land. The surface temperature is necessarily highest along the equator, or rather along a belt, varying from 2° to 8°, on either side of the equator, and then gradually diminishes as we approach either pole, but on the whole more rapidly in the southern than in the northern hemisphere. Along this equatorial zone temperatures have been found ranging from 78° to 85°-higher exceptional temperatures (87° and 88°) having occasionally been taken in parts of the Indian Ocean and the Gulf of Mexico. Though varying in surface temperature according to latitude—from 80° at the equator to perpetual ice towards either pole-it has yet been found that at very great depths the ocean preserves a uniform mean temperature of about 39. Thus, according to the experiments of Sir John Ross, the circle of the mean temperature of the ocean in the southern hemisphere lies between the 56th and 59th parallels of latitude; along which belt the uniform temperature of 391° has been found to prevail at all depths, from the surface downwards. To the south of this line, owing to the absence of solar heat, the surface depths are colder, and the mean of 39° is not reached in the 70th parallel till we descend to the depth of 4500 feet, beneath which to the greatest depths the temperature is uniformly at 390, while the surface temperature is only 30°. To the north of the line of mean temperature, in consequence of the absorption of the sun's heat, the surface - depths are warmer; and in the 45th parallel the mean of 39° is not reached till we descend 3600 feet; while at the equator we have to descend 7200 feet before the same mean is obtained, and then at all depths below this it maintains the unvarying mean temperature of 3910, though the surface is at 80°. Presuming that a similar order prevails in the northern hemisphere, we have thus three great regions of oceanic temperature-an equatorial and two polar, the former characterised by warm water at the surface, and the latter by cold.

121. Such are the facts, so far as ascertained, respecting the general temperature of the ocean; but inland seas and currents may be

colder or warmer according to the position they occupy and the direction from whence they proceed. The surface temperature of the Gulf of Mexico, for example, is several degrees warmer (86° and 88° have been named) than the main Atlantic under the same latitude; the waters of the Gulf Stream are also several degrees higher in a large portion of its course than those through which it flows; while the Arctic Current, on the other hand, is considerably colder. "It may be regarded as a general rule," says a high authority, "that the temperature of all inland seas, at great depths, represents nearly the mean temperature of the earth in the latitudes where they are situated; whilst in the ocean, the low temperature at the bottom, in every latitude, is produced by the cold currents setting eternally from the polar regions, and which maintain the water at an almost constant temperaturethat of its maximum density, 39° Fahr.”

122. Besides the preceding conditions of saltness, density, depth, and temperature, there are also those of colour and luminosity, usually adverted to by navigators and geographers. In small quantities water is generally regarded as colourless, but that of the ocean assumes different hues, and this altogether independent of the colours of the sky which may be mirrored on its surface. Thus, in the open ocean, shallow water is indicated by a green tint of different degrees, while profound depths are characterised by an indigo blue. Whether this arises from the greater density of the mass, or from some peculiarity of its saline constitution, is not yet satisfactorily known; but the fact remains, and “ seagreen waves" and "dark-blue oceans are something more than mere poetic fancies. Of course, in some localities there may be accidental or even permanent discolorations, arising from the entrance of river-water, from peculiarity of bottom, or from the presence of countless myriads of vegetable and animal organisms; hence the application of such terms as Red, Black, White, Yellow, Green, and Vermilion to certain seas and areas of the ocean. In general, however, the ocean water is clear and limpid, and, under favourable circumstances, objects are reported to have been seen at a depth of 300 and 400 feet, or about half the distance to which the sun's light is supposed to penetrate into the abyss of

waters.

123. The phenomenon of luminosity or phosphorescence is less general, perhaps, and seems to depend in a great degree on locality, season of the year, and state of the weather. Luminous animalcules and creatures of various kinds appear to be the proximate cause of the phenomenon, which becomes more apparent in still, dark nights, and where the surface of the water is disturbed by the stroke of an oar, or the friction of a passing keel.

NOTE, RECAPITULATORY AND EXPLANATORY.

In the preceding chapter attention has been directed to the main natural divisions of the ocean; and to the composition, density, depth, temperature, colour, luminosity, and other physical characteristics of their waters. The divisions of the ocean into Pacific, Atlantic, Indian, Arctic, and Antarctic areas, are sufficient for the ordinary purposes of description; and the nomenclature of their various ramifications after some discoverer, position, adjacent country, colour, or other peculiarity, has been long established in general geography. Their further subdivision into zones, belts, regions, and the like, comes under the consideration of Climatology rather than of Hydrography; and the technicalities (bays, gulfs, straits, creeks, &c.) by which their minor sections are distinguished, have been already noticed in par. 42. Their dimensions are matters merely of measurement and calculation; their composition, depth, temperature, and the like, are subjects for experiment and observation.

Occupying different positions on the earth's surface, and having different configurations, the oceans and seas, properly so called, are necessarily characterised by different physical and vital conditions. The Pacific, almost shut out from the influences of the Arctic Ocean, is in a different condition from the Atlantic, which communicates freely with the north; while the Indian Ocean, encircled on three sides by land, and lying largely in the torrid zone, presents external conditions differing widely from either. All three open broadly to the great Southern Ocean, from which they receive the primary impulse of their tides and currents; but while the Indian and Atlantic are deep and free, the Pacific is largely obstructed by reefs and islands. It should also be borne in mind that the main oceans, like the great continents, lie meridionally, or at right angles to the equator, and are thus prevented, by the intervention of the land, from receiving in their winds, tides, and currents, the normal effect of the earth's daily rotation.

Though differing slightly in different areas, the composition of sea-water is, on the whole, very equable, and 31⁄2 per cent of saline ingredients may be set down as the general average. Along the courses of the trade-winds, in circumscribed seas, and other similar areas subjected to active evaporation, this percentage may be slightly increased; while in others it may be correspondingly decreased by periodical rainfalls, by the melting of polar ice, or by the influx of large and rapid rivers. The average density of sea