One of the most remarkable properties of seawater is a certain lumiuous appearance, which has been observed iu all seas, but appears in its greatest splendour between the tropics. In calm weather, when the water is moved by the motion of a vessel, the light assumes the form of brilliant stars, or round masses of a greenish hue, frequently eighteen inches in diameter. They float by the vessel in every part of the water which her bottom has touched, as deep as the lowest part of her keel, and form behind her a long and fiery train. At other times, when the breeze is strong, and the billows break and foam, the light appears like fields of flashing fire, through which the vessel is making her way. When the night is dark, the brilliancy of the water forms a beautiful contrast with the black concave of the sky; but as soon as daylight returns, the splendour disappears, and the sea exhibits only its usual dingy colour. At night, the slight agitation of the water occasioned by the action of a steady breeze upon the surface is generally sufficient for producing it. Wo are entitled to believe, as the result of modern scientific research, that this phosphorescence of the ocean depends exclusively on tho presence of myriads of marine animals and animalcule, having the power of producing animal light (as animals higher in the scale of organisation produce animal heat); unless, indeed, the conjecture of Professor Schonbein, that it partly arises from the oxidation of dead organic matter in the sea by means of ozone, should hereafter be verified. The luminous animals belong to every sub-kingdom of invertebrate animals : Protozoa; Radiata, the Aealephm; Crustacea, as a class of Artnulosa ; Mollusca; and the esculent groups of the Annelida and ,Tunicata; some of the compound forms of the latter, or compound Aseidians, being among the most splendidly phosphorescent. In the Transactions of the Berlin Academy of Sciences is a valuable paper by Ehrenberg on the luminous animals of the sea, containing a copious list of the species observed to possess this property. [LIIMM'OSITY OE' ORGANIC BEINGS, in NAT. HIST. Dry.] From the well-known laws of gravitation, it is inferred that the surface of the sea is always at the same distance from the centre of the earth, and that consequently it forms a uniformly regular curve. This surface of course maintains the same level, and it is consequently the best basis from which to determine the relative elevations of the different parts of the land. Some facts bearing on this subject, and on the precautions requisite in the use of the sea-level as a datum line, are stated near the end of this article, where the physical changes of the sea are noticed. But though the surface of the sea is a regular curve, minute investigation has shown that there are some irregu larities, and that some parts of the sea are more elevated than others. This is particularly the case with closed seas, which are generally more elevated than the ocean. The level of the closed seas is higher than that of the ocean when the mass of water brought to them by the rivers which discharge into their basin is greater than that which is lost by evaporation, and the straits by which they are united to the ocean are not wide enough to carry off the surplus waters quickly. On the other hand, when the evaporation is greater than the supply of water from rivers, the level of the closed sea sinks below that of the ocean, and it must be supplied with water from the latter by the straits which unite them.
The Baltic, though of no great extent, and though united to the open sea by three straits, one of which is of considerable width, receives so great a supply of river-water, that its level is higher than that of the North *Sea. Very exact measurement has shown that this difference amounts to more than a foot between the level of the North Sea near the mouth of the river Eider and that of the Baltic near the town of Kiel. It is true that when the level of the North Sea has been raised by a continuance of western and north-western winds, a current sets from the Cattegat into the Baltic, but in calm weather it is always found that the current sets northward through the three straits. The difference of level between the Black Sea and the Mediter ranean is much greater. The large rivers which fall into the Black Sea bring down an immense volume of water ; and accordingly a very strong southern current is constantly found to be setting southward through the Strait of Constantinople into the Sea of Marmora. It generally runs with a velocity of about three miles an hour, which however at one place, called the " Devil's Current," is much greater, and at times between five and six miles per hour. The velocity of this current must vary with the seasons; for it is stated that the level of the Black Sea in winter is between two and three fathoms higher thau in summer. The Sea of Marmora, which thus receives the surplus of the waters of the Black Sea, must also be more elevated than the Mediterranean ; for the current which sets through the Strait of the Dardanelles is likewise constant and rather quick, though not so quick as that in the Strait of Constantinople.
The Mediterranean, on the other hand, receives a very scanty supply of water by rivers; for with the exception of the Nile, no large stream falls into its basin, which is of much greater extent than that of other closed seas, and therefore it must lose a great volume of water by evaporation. Halley showed that the Mediterranean, whose tempera ture is from 4' to 5' Fahr. higher than that of the Atlantic under the same latitude, must lose by evaporation nearly three times as much water as is brought into it by the rivers. The deficiency is supplied in two ways: by the current of the Dardanelles, which brings to it the surplus waters of the Black Sea and of the Sea of Marmora; and by that which seta through the Strait of Gibraltar from the Atlantic Ocean. The Atlantic current runs somewhat more than one mile and a half per hour. It has been supposed that, though this current con stantly sets into the Mediterranean, an under-current runs in an oppo site direction, carrying back a portion at least of the water to the Atlantic ; but the attempts which have been made to establish this fact have failed, and it is probable that this supposed undercurrent does not exist. Notwithstanding the large supply of water which the Mediterranean receives at its two extremities, the level is below that of the Atlantic. Corabceuf found that the difference of level on the Mediterranean near Perpignan, and on the Bay of Biscay near Bayonne, amounted to nearly six feet ; and Delambre and 3lecbain found it to be nearly three feet between the North Sea at Dunkerque and the Mediterranean near Perpignan.
In these instances the difference of level is satisfactorily explained;; but the explanation is not so easy with respect to the great differ ence between the level of the Mediterranean and that of the Red Sea. These two seas are separated by the Isthmus of Suez, which extends about 70 miles from north to south. When the French occupied Egypt, they executed an extensive levelling across this isthmus ; and the result was, that the Red Sea is above 32 feet higher than the Mediterranean. No river of importance, not even a perennial stream, falls into the Red Sea, which must also lose a con siderable volume of water by evaporation. This loss of water is probably supplied by the current which sets into the Red Sea from the Indian Ocean ; and some persons are of opinion that the difference of level between the Red Sea and the Mediterranean might be pro duced by this current. But it seems improbable that such an effect can be produced by this cause. According to Horaburgh and Wellsted (` Journ. of Lend. Geogr. Soc.,' voL vi.), a current sets from the Indian Sea into the Red Sea between October and May, and it often runs with great rapidity. But between May and October the northern winds prevail through the whole extent of the Red Sea ; aud these winds, which frequently blow a gale, cause a continual current to set through the straits into the Gulf of Aden. Under such circumstances, it is evident that the sea must fall to its natural level, especially as this state of things continues for more than three months. Wellsted observes that in this season, from May to October, the reefs in the northern part of the Red Sea have about 2 feet less water on them than in the remaining months of the year. This therefore appears to be the whole extent of the difference produced on the level of the Red Sea by the current, which enters it through the Strait of Bab-el Mandeb from October to May ; but the French, as already observed, found the difference between the levels of the Red and Mediterranean seas to be not less than 32 feet. It would therefore appear that the Indian Ocean itself must be about 30 feet higher than the Mediter ranean, and probably also higher than the Atlantic near the Strait of Gibraltar (see below) and that the difference of level in the different parts of the ocean is much greater than is commonly supposed. This is also proved by the difference of level between the Atlantic and the Pacific on both sides of the Isthmus of Panama. Accord ing to Lloyd (` Phil. Trans.,' 1830), the mean rise and fall of the Pacific two days after full moon is 21.22 feet, and in the Caribbean Sea 116 feet. The water at high-water mark in the Atlantic is 13'55 feet lower than in the Pacific. The mean between the high and low water in the Pacific is 10.61 feet, and in the Atlantic 0'58. It would therefore appear that the level of the Pacific is feet higher than that of the Caribbean Sea, as at low-water, two days after full-moon, the Pacific sinks G.51 feet below the level of the Atlantic ; but it rises at high-water feet above it.