Though the saltness of the sea considerably aids in preserving it from putrefaction it is not alone sufficient for that purpose, and without the constant motion produced by winds, tides, and currents, it would in a short time corrupt, as has been frequently experienced in Jong calms within the tropics. The general color of the sea in the open ocean is a deep greenish blue': the latter tint, which is predominant, seems to proceed from the same cause that gives an azure color to the atmosphere, and a deeper blue to distant mountains; for, the blue rays being the most refrangible, are reflected in greatest quantity by the aquatic fluid, which, by reason of its density and depth, causes them to undergo a strong refraction. The other shades, which have been observed in the waters of different seas, seem to depend on local causes, and often perhaps on optical illusion; thus the water of the Levant is said at times to have a purple tint, the sea in the Gulf of Guinea to be whitish, and near the Maldiva Islands black. The water of the Gulf of California is reddish, whence its name Vermilion Sea. The changes of color in proceeding from the British Sea to the Frozen Ocean have been noticed by several voyagers; in the first it is the common greenish blue, in the sea of Norway a clear deep blue, and in the Frozen Ocean a deep black. The approach to the coasts of the continents, or of large islands, is generally denoted by the lighter green or yellowish tint of the water denoting being in soundings. The southern seas present at times a phenomenon which terrified their early navigators, who, seeing large spaces of the sea of a blood color, conceived it a portent of some dreadful catastrophe; this appearance, however, seems to be generally produced by a multitude of sea insects of a red color. That the sun's rays rarely penetrate below the depth of forty-five, or according to some of 113 fathoms, below which the sea receives no light, and consequently little or no direct heat from the sun; hence it is inferred that the temperature of the bottom must follow that of the interior of the globe in the different latitudes. 2dly, That the temperature of the sea at the surface differs from that of the atmosphere plus or minus, according to the circumstances of locality, season, weather, time of the day, &c. And 3dly, That the temperature decreases with the depth to a certain degree, but never to freezing, which is prevented by the constant internal heat of the earth. The following series of observations on the specific gravity and temperature of sea-water, by Dr. Davy, inserted in the Edinburgh Philosophical Journal, were made during a voyage homeward from Ceylon, in 1819 and 1820. They embrace a very large portion of the earth's surface, and, from the accuracy and sagacity of the able chemist by whom they were made, they will be regarded by philosophers as forming a valuable addition to our hydrographical knowledge. The series of experiments commences at Columbo in Ceylon, on the 4th of December, 1819; and between the 1st of February and the end of June 1820: they were made on board the Eclipse. 9h A.M. 10h A. M. 12 Air. Water. Wind and Weather.. 2 P. M. 5 79 81 8 80.5° These temperatures land. N.E. gentle, pretty clear. February 5th 1820. N. lat. 4° 10′, E. long. 80° 15′. Air. Water. Ih A. M. 8 The luminous appearance of the sea at night is a magnificent and imposing spectacle. When gently agitated, innumerable sparkles of light, some of dazzling brilliancy, others of a silvery white, will group themselves in a thousand forms; but when disturbed the appearance is more tumultuously grand, waves of fire rising, rolling onward, and breaking in brilliant foam. This phenomenon has occupied the attention of many naturalists, some ascribing it solely to animals of the zoophite and mollusca classes, all of which they say possess phosphorus, in a greater or less degree; others, while they admit the existence of luminous sea insects, are of opinion that the light of the sea is more particularly caused by animal and vegetable substances, which in the process of putrefaction discharge 10 their phosphorus. Sir Isaac Newton was inclined to attribute it to friction alone, from the observation that the light is more brilliant when the sea is most agitated; others again, from particular experiments, conceive that it may proceed, at least in part, from a matter contained in the sea-water which has a direct analogy with electricity; finally, it has been supposed that the spawn of fish has a considerable share in this phenomenon. See our article MEDITERRANEAN. The observations made on the temperature of the sea afford the following general results. 1st. 12 Wind. 2 P. M. 4 80 82 Calm, and overcast. There was much thunder and lightning during the night, and the weather was squally with heavy rain. February 8th 1820. S. lat. 0° 5′, E. long. 81° 37'. Air. Water. Wine and Weather. Do. gentle. 6дh A. M. 79° 83 Do. do. do. Do. do. do. Calm, do. 6h P. M. 81-5 83-75 Calm, clear. N. W. Almost 81.5 82.5 5.5 S. W. do. clea The night was rather squally, with some heavy showers. Between about N. lat. 3° and S. lat. 7° or 8° a north-west or westerly wind prevails during the same month that the north-east wind prevails in Ceylon. About the limits of the north-east and north-west, and of the south-east and northwest, calms commonly occur. Hence, we may expect that, as those parts of the ocean within the tropics are particularly liable to calms, the calm, temperature of the water will be unusually high; and that in those parts of the ocean within the tropics which are particularly liable to squalls, the temperature of the water will be lower. The north-west or little monsoons blow from Madagascar, and the direction is probably connected with that great island. The north-west is succeeded by the south-west monsoon in the above latitudes. clear. 81° 49'. Air. Water. Hygr. Wind and Weather. 6th A. M. 77° 82-5° W. very gentle, clear. W by S. do. do. W. gentle do. 6° W..by N. do. do. 8 80 83 10 81 83 12 81 84 4 N. W. squally, raining Do. do. cloudy. 79 82 3 Do. do. do. The night was rather squally and cloudy, with occasional showers. These hygrometrical observations were made with two thermometers, one of which had its bulb covered with an absorbent substance, and wetted with water. The degrees in the column show the descent of the mercury by the cold, produced by evaporation. February 10th 1820 S. lat. 2° 8', E. long. Air. Water. Hygr. Wind and Weather. 6th A. M. 80° 83.5° 3.5° S. W. very gentle, 80 83.5 5 cloudy. Do. gentle do. 83.5 84.5 7.5 Calm, light clouds 84 4 S. fresh, a squall cloudy, showery. 12 78.5 82 3.5 Do. do. 10 5.5 Do. do. 12 Do. do. 2 P. M. 80 4 The night was boisterous, with heavy rain, and some thunder and lightning. February 11th 1820. S. lat. 4° 45', E..long. 81° 54'. 82 84 6 81.5 83.5 3 approaching. Calm, light clouds. S. by E. moderate. Till midnight there was a fresh breeze, which was followed by a calm and much rain. 9° 3', E. long. 81°. Wind and Weather. S. E. very gentle, pretty clear. Do. do. do. Do. do. do. Do. do. do. N. gentle, dark clouds. Calm, some dark clouds. The night was calm and fine. 80° 39'. Air. Water. Hygr. 6дh A. M. 80° 83.5° 4° 10 12 2 P. M. 6 8 Wind and Weather. 83 a shower. 81.5 84 4.5 Do. do. The night was rather fine, and the wind occasionally approaching to fresh. February 18th 1820. S. lat. 10° 8', E. long. 80° 29'. The night was stormy and rainy, and the wind blowing a gale. During this gale the sky was thickly overcast, so as to be of a dark gray or light sooty hue, but the sea retained its usual color. Its blue color appeared very distinct, when one looked immediately down from the ship into the sea, and it was equally evident in the waves as they rose, their heads 'being between the light and the eye of the observer. Even in the color of the surface of the sea in general a tint of blue might be distinguished, but it was not bright on account of the darkness of the surface. Hence we may infer that the ocean does not owe its blue color to the reflected azure of the sky, as several authors have supposed. It was long doubted whether sea-water would freeze, and this doubt was strengthened by the experience of navigators, who found that the ice taken up from the ocean, when thawed, produced perfectly fresh water: hence Buffon supposed it to be formed in rivers, by whose currents it was carried into the ocean. Captain Cook on the contrary ascribed its origin to snow; which, being more solid, and at the same time lighter, bulk for bulk, than the sea-water, would float on the surface and be converted into ice, which must continually augment in thickness from other snow, rain, &c. It is now, however, proved that it requires no very extraordinary degree of cold to freeze water more impregnated with salt than that of the ocean, and that it gets rid of its salt in the process of congelation. latitudes of the southern than of the northern Oceanic ices are generally met with in lower hemisphere; for the northern polar sea being almost surrounded by land, which opposes the free drift of the ices, it is only the pieces formed in the bays of the rivers of America that are carried by the polar current to the south, and such are occasionally met with on the west side of the Atlantic, so low as the latitude of 40°. On the S. by E. do. slightly contrary, the Greenland ships who keep along the coast of Europe, where is a constant current from the south, seldom meet with ice till they arrive at the latitude of 76°, and they are usually enabled to advance to the latitude of 80° or even of 82° before their progress is finally stopped by connected field ice. Between Asia and America captain Cook found the continents joined by ice in 70°, and his farthest progress was only 70° 48′. cast. S. S. W. do. do. overcast. S. W. do. overcast. S. moderately do. The night was fine with little wind. In the southern seas, there being no obstacle from lands to the drifting of the ices, they are often met with in large masses in latitude 40°. In 60° the ice islands are so numerous as to render navigation extremely perilous, and connected field ice usually entirely arrests it in 70°. In this ocean captain Cook was unable to approach the pole nearer than 71° 10′. This navigator also describes the ice islands of the southern seas as of much greater extent and elevation than those he ever met with in the northern. It is observed that the atmosphere is warmer where ice islands are first met with than in the immediate lower latitude previously passed through, which seems to proceed from the ice reflecting the sun's rays, and also that the temperature is greater near these masses in the rs gions of their formation than when having drifted into lower latitudes they are thawing, which is evidently caused by the progress of fusion; for, ice being formed by the deprivation of caloric, its fusion can only be produced by a new combination of the same element, which it absorbs from the atmosphere, and renders it extremely cold. The approach to ice islands is denoted, even in the darkest night, by a whitish light which they reflect in the horizon, and which seamen call the blink. nomena. Humboldt has some observations on this subject, which combine, in his own manner, a great deal of scientific and practical information:'We must distinguish,' he says, 'with respect to the temperature of the ocean, four different phe1st, The temperature of the water at the surface corresponding to different latitudes, the ocean being considered at rest, and destitute of shallows and currents. 2dly, The decrease of heat in the superimposed strata of water. 3rdly, The effect of billows on the temperature of the surface water. 4thly, The temperature of currents, which impel with an acquired velocity the waters of our zone across the immoveable waters of another zone. The region of warmest waters no more coincides with the equator, than the region in which the waters reach their maximum of saltness. In passing from one hemisphere to another, we find the warmest waters between 5° 45′ of N. lat., and 6° 15′ of S. lat. Perrins found their temperature to be 82.3°; Quevedo 83.5°; Churruca 83.7°; and Rodman 83-8°. I have found them in the South Sea to the east of the Galapagos Isles 84-7°. The variations and the mean result do not extend beyond 1.3°. It is very remarkable that, in the parallel of warmest waters, the temperature of the surface of the sea is from 3.6° to 54° higher than that of the superincumbent air. Does this difference arise from the motion of the cooled particles towards the bottom, or the absorption of light which is not sufficiently compensated by the free emission of the radiant caloric? As we advance from the equator to the torrid zone, the influence of the seasons on the temperature of the surface of the sea becomes very sensible; but, as a great mass of water follows very slowly the changes in the temperature of the air, the means of the months do not correspond at the same epochs in the ocean and in the air. Besides, the extent of the variations is less in the water than in the atmosphere, because the increase or decrease in the heat of the sea takes place in a medium of variable temperature, so that the minimum and the maximum of the heat which the water reaches are modified by the atmospherical temperature of the months which follow the coldest of the warmest months of the year. It is from an analogous cause that in springs which have a variable temperature, for example near Upsal, the extent of the variations of temperature is only 19.8°, while the same extent in the air from the month of January to August is 39-6°. In the parallel of the Canary Islands Baron Von Buch found the minimum of the temperature of the water to be 68°, and the maximum 74.8°. The temperature of the air in the warmest of the coldest months is, in The excess in the mean temperature of the water over that of the air attains its maximum beyond the polar circle, where the sea does not wholly freeze. The atmosphere is cooled to such a degree in these seas (from 63° to 70° of lat., and 0° of long.) that the mean temperature of several months of winter descend on the continents to 14° and 10·4°, and on the coasts to 23° and 21.2°, while the temperature of the surface of the sea is not below 32° or 30-2°. If it is true that even in those high latitudes the bottom of the sea contains strata of water which, at the maximum of their specific gravity, have 39.2° or 41° of heat, we may suppose that the water at the bottom contributes to diminish the cooling at the surface. These circumstances have a great influence on the mildness of countries in continents separated from the Pole by an extensive sea. The superior degree of cold of the southern hemisphere than of the northern, in equal latitudes, is now generally ascribed to the greater extension of the southern ices towards the temperate zone. The absence of any considerable land in the high southern seas, and the form of the continents, which terminate in angular points, leave a free course to the polar currents, and permit them, as we have already observed, to convey the ices of the pole far into the temperate zone, where their presence causes those sudden transitions from heat to cold, and those intense fogs, met with by navigators in the great southern ocean. Astronomers have also attributed the superior cold of the southern hemisphere to the sun's being seven days and eighteen hours less in the southern than in the northern signs; but the difference produced by this cause ought not to exceed the one twenty-fifth, while the real difference appears to be one-seventh. The theory of the rays of heat has afforded another explanation in the attempt to demonstrate that in a given time the southern hemisphere loses a greater proportion of its constant proper heat than the northern: but this cause ought not to cease suddenly, between 35° and 40° of lat., as is observed to be the case. In several places near the shores of the sea springs of fresh water are observed, rising to the surface, and refusing to combine with the salt water that surrounds them. The most remarkable instances of these phenomena are in the gulf of Spezia; in the Persian Gulf, near the isles of Bahrein; and in the bay of Xagua, on the south coast of Cuba. It may be presumed that veins of water, finding no outlet towards the surface of the land, follow the direction of internal fissures even under the sea, until they meet with such an outlet, through which they naturally ascend, with a force in proportion to the' elevation of their sources and the declivities of the subterraneous canals, in the same manner as spouting springs on the land. The waters of the ocean cede to very slight impulsions, and are constantly agitated by three different movements; 1. The undulatory movement, or waves; 2. The sidereal movement, or tides; and 3. Currents. equatorial region performing its diurnal revolution with greater velocity, and at the same time tracing a larger circle than any other portion of the globe, its waters must be more agitated by the centrifugal force, which, depriving them of a portion of their gravity, renders them more susceptible of external impressions. It is doubtful to what depth the sea is agitated by the winds: twenty fathoms seem to be the farthest to which divers have descended, and at this depth they have found the water so troubled, that mud and shells were carried to considerable distances; Every movement of the atmosphere, in the while other divers pretend that at the depth of form of winds, produces a correspondent move- fifteen fathoms they have found the water perment on the surface of the ocean, which increases fectly tranquil even in the greatest storms. The in rapidity and violence with the velocity and breaking of the waves on a flat shore is named a force of the winds: thus a moderate breeze pro- surf, and is a phenomenon, worthy of particular duces a gentle undulation, which, moving slowly notice. The surf, says Marsden, (History of onwards, exhausts itself and subsides tranquilly. Sumatra), is at times composed of but one rank In a storm the ocean is furrowed by tremendous of waves along the shore, at other times there are waves or mountainous ridges of water, each of two, three, four, and even more, one behind the which rolls on with furious rapidity, until its other, extending half a mile from the shore. The summit arrives at an overcharging elevation, surf begins to take its form at some distance from which it necessarily precipitates itself by from the place where it breaks, and augments by the force of gravity, and, by the acceleration it degrees as it advances, until it arrives at the has acquired in its descent, impels forward the common height of fifteen or twenty feet, from mass of water immediately before it, which in its the summit of which elevation the wave precipiturn rises, forms a wave, and pushes forward the tates itself like a cascade from a precipice, with water before it, and thus is a continual succes- a noise that may be heard at several miles dission of waves generated. Dr. Woolaston found tance, and which often in the night gives the nathe velocity of the waves to be nearly sixty miles vigator sufficient notice of his danger to enable an hour close to the east coast of England. It him to escape shipwreck on shores where there seems of necessity that their comparative velo- would be no human possibility of saving his city must be greater in the open ocean than near life. Although from the first formation of the the shores in shoal water, where the mixed par- surf the water seems to have a rapid progressive ticles of sand and mud increasing the density of motion towards the land, yet a light object floatthe fluid, as well as the friction on the bottom, ing on it, instead of being carried on shore, drifts must considerably retard their progress. parallel to it if the tide is flowing, and drives off if it is ebbing; whence it seems probable that the movement is propagated in the fluid alone as sound is in the air, and that the mass of the wave is not propelled forward, the only real progressive movement being produced by the perpendicular fall at the moment of the breaking of the surf, when the wave, by its descending weight, spreads itself in foam to a greater or less distance, in proportion to its elevation and to the declivity of the shore. Though the wind produces the wave which is to form the surf, it is' certain that the wind is not the immediate cause of the surf itself, for it is often greatest in a calm, and least in a storm, and is also often most violent when the wind blows off the shore. Marsden supposes that when the wave approaches a shore whose depth is not in proportion to its volume, this wave, instead of pressing on a mass of water which would elevate itself to an equal height and form another wave, presses on the ground, the reaction of which forces it to precipitate itself as we have described. The greatest surfs are observed between the tropics, and particularly on the coast of Africa, on that of Coromandel, and the west coast of Sumatra. Geometricians have attempted to calculate the velocity of the propagation of waves, as being the same as that which a heavy body acquires in descending from a height, equal to half the depth of water in the channel; consequently, if the depth is one foot, the velocity of the wave will be 5 feet per second of time, and, as the depth is greater or less, the velocity will vary into a ratio underdoubled of the depths, provided they are not too great.-La Grange Mécanique Analytique. To that state of the ocean after a storm, when mountainous and long billows follow each other slowly and subside without breaking, seamen give the name of swell, while high breaking waves are called a sea. After a storm, if a wind springs up in an opposite direction, the waves it creates being contrary to the swell, what is called a cross sea is produced, often more dangerous than the most mountainous but regular waves. A swell is often experienced in a contrary direction to the wind blowing at the moment, and is a certain prognostic of a change of wind to the direction of the swell, the latter sometimes preceding the former several hours. The extraordinary long swell, always met with near the equator, seems to be caused by the more direct and powerful influence of the sun and moon on this part of the ocean, by which it is necessarily more agitated than nearer the poles, where the action of the heavenly bodies is indirect; besides, the Tides are periodical oscillations of the sea, caused, as it is said, by the attractions of the sun and moon, and more particularly of the latter; but they involve considerations of sufficient importance to entitle them to a distinct notice. See TIDES. |