of the water is usually more elevated than in the latter. This seems to be more especially the case if the only openings of these gulfs are towards the east; and it is attributed with reason to the accumulation which arises from the water being driven into these confined inlets by the general movement to which allusion will be made presently. When the French engineers were in Egypt, they made observations, according to which the waters of the Red Sea, on the east side of the Isthmus of Suez, were 32 feet higher than those of the Mediterranean, on the opposite shore of the same isthmus. M. Humboldt made observations of a similar kind upon the Isthmus of Panama, and his conclusion is, that the waters of the Gulf of Mexico are from 20 to 23 feet higher than those of the Pacific on the other side. Of certain inland seas the level varies with the seasons; the Baltic and the Black Sea, which are in fact almost lakes, swell in the spring, from the abundance of water brought down to them at that period by the rivers. The general colour of the sea is a deep bluish green, which becomes clearer towards the coasts. This colour is thought to arise entirely from the same cause as the azure tint of the sky; the rays of blue light, being the most refrangible, pass in the greatest quantity through the water, which, on account of its density and depth, makes them undergo a strong refraction. The other colours, exhibited in parts of the sea depend on causes which are local, and sometimes deceptive. The Mediterranean, in its upper part, is said to have, at times, a purple tint. In the Gulf of Guinea the sea is white; around the Moldavia Islands it is black; and in some places it has been observed to be red. These appearances are probably occasioned by vast numbers of minute marine insects, by the nature of the soil, or by the infusion of certain earthy substances in the water. The green and yellow shades of the sea proceed, frequently, from the existence of marine vegetables at or near the surface. The water of the sea contains several extraneous substances, in proportions varying in different places. The component parts, in addition to pure water, are commonly muriatic or marine acid, sulphuric acid (vitrol), fixed mineral alkali, magnesia, and sulphurated lime. By boiling, or evaporation in the air, common salt (muriate of soda) is obtained, which for salting meat is preferred to the salt of springs. The saltness of the sea appears, with some local exceptions, to be less towards the poles than near the tropics; but the difference is very slight, and perhaps the observations made are not sufficiently numerous to justify any positive general conclusions. Some observations which have have been made tend to prove that the sea is less salt at the surface than towards the bottom. The degree of saltness, in particular parts of the sea, frequently varies from temporary causes. The violent tropical rains have an effect in diminishing it, especially near coasts, where an increased volume of fresh water is brought down by the rivers. The Baltic is, at all times, less salt than the ocean, and, when a strong east wind keeps out the North Sea, its waters are said to become almost fit for domestic uses. The most curious phenomenon of all is that of springs of fresh water rising up in the midst of the sea; Humboldt mentions that in the Bay of Xagua, on the southern coast of Cuba, springs of this kind gush up with great force at the distance of two or three miles from the land. The bitterness which exists in sea water, but apparently not beyond a certain depth, is, with much probability, considered to be owing to the vegetable and animal matter held there in a state of decomposition. Water being a bad conductor of heat, the temperature of the sea changes M&M much less suddenly than that of the atmosphere, and is by no means subject to such extremes as the latter.* It may safely be affirmed that the tempera ture never, in any season, or under any latitude, exceeds 85° or 86° of Fah renheit. The existence of banks or shallows has a local effect in diminishing the temperature of the ocean, but the great agents in modifying it are currents, which mingle together the waters of different depths and regions. Thus the Gulf Stream, as it is termed, which sets into the Gulf of Mexico, is much warmer than the neighbouring parts of the sea: the current of Chili is just the reverse. TABLES OF THE TEMPERATURE OF THE ATLANTIC OCEAN, IN DIFFERENT DEGREES OF LONGITUDE. The effect of the sea is to equalize temperature, so that a maritime country is not liable to such extremes, either of heat or cold, as an inland one. The sea itself being of a very equable temperature, the winds which pass over an extent of it partakes somewhat of the same character. When a cold wind passes over sea it receives part of the warmth of the water, the upper particles of which being thus rendered cooler, and consequently heavier than those below, descend and are succeeded by warmer particles; so that there is a continual tendency in the sea to temper a cold wind passing over its surface. A cold wind, blowing overland, is at first rendered warmer by the earth's surface; but this surface quickly becoming cooled, ceases to have any effect upon the wind, which, therefore, travels on with undiminished rigour. Again, a warm wind in passing over the sea, is cooled by the agitation which it produces, bringing up cooler water from below, as well as by the constant evaporation which it occasions: the surface of the water also cannot, as that of the land, be powerfully heated by the sun's rays, because it affords them a free passage, and therefore it cannot communicate heat to the atmosphere in the degree which the land does. From these circumstances it results that, though a place situated inland and another upon the coast may have the same mean annual temperature, the range of the thermometer at each will be very different; the summers of the latter will be cooler, and the winters milder than those of the former. It is from this cause that islands are so much more temperate than continents. It follows, too, that countries in our hemisphere will be rendered warmer by having large tracts of land to the south, and sea to the north; and cooler when the relative position of these two is reversed. This fact is exemplified by a comparison of the climate of India with that of Africa, north of the equator, the heats of the former country being much more supportable than those of the latter. Not only the temperature of a wind, but also its degree of moisture, depends upon the nature of the surface over which it passes. A wind coming up from the ocean is loaded with vapours, but one sweeping over an extent of land is rendered dry and parching. This explains to us why, in our own island, a south-west and an easterly wind are so opposite in character. These tables refer only to the Atlantic ocean; but the experiments which have been made in the South sea, and in the Indian ocean, show that, within a certain distance of the equator, the general temperature of the sea follows nearly the same rule in corresponding latitudes. Within the tropics there is no sensible difference in north and south latitudes; there is very little even as far as 35° and 40°; but, when we advance into high latitudes, there can be no doubt that the sea is colder in the southern than in the northern hemisphere. Ice extends from five to eight degrees of latitude farther from the south than from the north pole, owing, it is probable, to the almost entire absence of land near the Antarctic circle; while the north pole is so nearly surrounded by land, that the ice of the Arctic ocean is shut up, and cannot be carried forward to such a distance by the current which sets towards the equator. Bays, inland seas, and the spaces among clusters of islands, where the action of the waves is more confined, and the water usually of less depth, are the most favourable places for the production and accumulation of the marine ice. It is on this account that the navigation of the Baltic is annually stopped by the ice, in a latitude not more northerly than that of the tracts which, in the main ocean, are always open to the passage of ships. In severe winters, people may travel in sledges across the entrance of the Gulf of Bothnia (latitude 60°) which, including the numerous small intervening islands, is a distance of 115 miles. The body of ice accumulated in Lancaster's Sound has defied all the attempts that have been made to accomplish the north-west passage from the Atlantic to the Pacific. ances. The ice of the polar seas assumes a great variety of shapes and appearThe vast and thick sheets, which are met in the high latitudes, are called fields by navigators; they are so extensive that their boundaries cannot be seen from a ship's mast-head; and Captain Cook found a chain of them joining Eastern Asia to North America: sheets of less extent than fields are called floes. Bergs are islands of ice, considerably elevated above the water; and, though of the most varied forms, commonly perpendicular on one side, and sloping gradually down on the other-in height they are sometimes as much as 200 feet. There are two ways of explaining the formation of these bodies. The large masses of ice in the Polar seas, when crowded together by winds and currents, exert such an enormous pressure upon each other, that they are frequently broken, and the fragments are piled up so as to form mounds and ridges of considerable elevation; it is thus that many of the small icebergs originate. There is no doubt, however, that the most bulky of these bodies are detached portions of vast glaciers, such as abeund on the precipitous coasts of Greenland and Spitzbergen, broken off in consequence either of their own weight, or the undermining actions of the waves, and then carried by winds and currents to other parts of the ocean. When the summer has well advanced, the masses of ice, which have been frozen together during the winter, gradually separate, and clear spaces of water are left. As soon as the end of September, these open spaces again begin to freeze over; and, before this effect commences, the temperature of the air must be very much lowered, owing partly to the freezing point of sea being 34° (Fahrenheit) below that of common water, but more especially to the surface which the water presents to the atmosphere being repeatedly changed, before its temperature is sufficiently reduced for it to freeze. This change in the surface, which is greatly assisted by the agitation of the sea, takes place in consequence of the particles of a liquid body becoming specifically heavier as they get cooled, so that they descend and are succeeded by warmer particles. There are three kinds of movements constantly going on in the waters of the sea-1. The agitations which its surface undergoes by the action of winds.-2. Tides, which are the result of the attraction exercised on the water by the sun and moon.-3. Currents, which arise from different causes. some of them existing within the element itself. As the particles of a fluid press equally in every direction, it follows that when a portion of the surface of the water is displaced by wind, the adjoining water instantly rushes in to restore the equilibrium or balance which has been destroyed: this accounts for the formation of waves. When a violent impulse has been communicated, the waves continue in motion for some hours after the gale has entirely subsided, on the same principle as a pendulum continues to swing for some time after it has been set in action. Yet the agitation occasioned by winds extends to comparatively but a little way below the surface of the water; divers say that, in the roughest weather, it is calm at the depth of 90 feet. |