SEA
, in Geography, is frequently used for that vast tract of water encompassing the whole earth, more properly called ocean. But
Sea is more properly used for a particular part or division of the ocean, denominated from the countries it washes, or from other circumstances. Thus we say, the Irish sea, the Mediterranean sea, the Baltic sea, the Red sea, &c.
Sea among sailors is variously applied, to a single wave, or to the agitation produced by a multitude of waves in a tempest, or to their particular progress and direction. Thus they say, a heavy sea broke over our quarter, or we shipped a heavy sea; there is a great sea in the offing; the sea sets to the southward. Hence a ship is said to head the sea, when her course is opposed to the setting or direction of the surges. A Long Sea implies a steady and uniform motion of long and extensive waves. On the contrary, a Short Sea is when they run irregularly, broken, and interrupted, so as frequently to burst over a vessel's side or quarter.
1. General Motion of the Sea. M. Dassie of Paris, in a work long since published, has been at great pains to prove that the Sea has a general motion, independent of winds and tides, and of more consequence in navigation than is usually supposed. He affirms that this motion is from east to west, inclining toward the north when the sun is on the north side of the equinoctial, but toward the south when he is on the south side of it. Philos. Trans. No. 135.
2. Bason or Bottom of the Sea, or Fundus Maris, a term used to express the bed or bottom of the sea in general. Mr. Boyle has published a treatise on this subject, in which he has given an account of its irregularities and various depths founded on the observa tions communicated to him by mariners.
Count Marsigli has, since his time, given a much fuller account of this part of the globe. The materials which compose the bottom of the Sea, may reasonably be supposed, in some degree, to influence the taste of its waters; and this anchor has made many experiments to prove that fossil coal, and other bituminous substances, which are found in plenty at the bottom of the Sea, may communicate in great part its bitterness to it.
It is a general rule among sailors, and is found to hold true in many instances, that the more the shores of any place are steep and high, forming perpendicular cliffs, the deeper the Sea is below; and that on the contrary, level shores denote shallow Seas. Thus the deepest part of the Mediterranean is generally allowed to be under the height of Malta. And the observation of the strata of earth and other fossils, on and near the shores, may serve to form a good judgment as to the materials to be found in its bottom. For the veins of salt and of bitumen doubtless run on the same, and in the same order, as we see them at land; and the strata of rocks that serve to support the earth of hills and elevated places on shore, serve also, in the same continued chain, to support the immense quantity of water in the bason of the Sea.
The coral fisheries have given occasion to observe that there are many, and those very large caverns or hollows in the bottom of the Sea, especially where it is rocky; and that the like caverns are sometimes found in the perpendicular rocks which form the steep sides of those fisheries. These caverns are often of great depth, as well as extent, and have sometimes wide mouths, and sometimes only narrow entrances into large and spacious hollows.
The bottom of the Sea is covered with a variety of matters, such as could not be imagined by any but those who have examined into it, especially in deep water, where the surface only is disturbed by tides and storms, the lower part, and consequently its bed at the bottom, remaining for ages perhaps undisturbed. The soundings, when the plummet first touches the ground on approaching the shores, give some idea of this. The bottom of the plummet is hollowed, and in that hollow there is placed a lump of tallow; which being the part that first touches the ground, the soft nature of the fat receives into it some part of those substances which it meets with at the bottom: this matter, thus brought up, is sometimes pure sand, sometimes a kind of sa d made of the fragment of shells, beaten to a sort of powder, sometimes it is made of a like powder of the several sorts of corals, and sometimes it is composed | of fragments of rocks; but beside these appearances, which are natural enough, and are what might well be expected, it brings up substances which are of the most beautiful colours. Marsigli Hist. Phys. de la Mer.
Dr. Donati, in an Italian work, containing an essay towards a natural history of the Adriatic Sea, printed at Venice in 1750, has related many curious observations on this subject, and which confirm the observations of Marsigli: having carefully examined the soil and productions of the various countries that surround the Adriatic Sea, and compared them with those which he took up from the bottom of the Sea, he found that there is very little difference between the former and the latter. At the bottom of the water there are mountains, plains, vallies, and caverns, similar to those upon land. The soil consists of different strata placed one upon another, and mostly parallel and correspondent to those of the rocks, islands, and neighbouring continents. They contain stones of different sorts, minerals, metals, various putrefied bodies, pumice stones, and lavas formed by volcanos.
One of the objects which most excited his attention, was a crust, which he discovered under the water, composed of crustaceous and testaceous bodies, and beds of polypes of different kinds, confusedly blended with earth, sand, and gravel; the different marine bodies which form this crust, are found at the depth of a foot or more, entirely petrified and reduced into marble; these he supposes are naturally placed under the Sea when it covers them, and not by means of volcanos and earthquakes, as some have conjectured. On this account he imagines that the bottom of the Sea is constantly rising higher and higher, with which other obvious causes of increase concur; and from this rising of the bottom of the Sea, that of its level or surface naturally results; in proof of which this writer recites a great number of facts. Philos. Trans. vol. 49, pa. 585.
3. Luminousness of the Sea. This is a phenomenon that has been noticed by many nautical and philosophical writers. Mr. Boyle ascribes it to some cosmical law or custom of the terrestrial globe, or at least of the planetary vortex.
Father Bourzes, in his voyage to the Indies, in 1704, took particular notice of this phenomenon, and very minutely describes it, without assigning the true cause.
The Abbé Nollet was long of opinion, that the light of the Sea proceeded from electricity; and others have had recourse to the same principle, and shewn that the luminous points in the surface of the Sea are produced merely by friction.
There are however two other hypotheses, which have more generally divided between them the solution of this phenomenon; the one of these ascribes it to the shining of luminous insects or animalcules, and the other to the light proceeding from the putrefaction of animal substances. The Abbé Nollet, who at first considered this luminousness as an electrical phenomenon, having had an opportunity of observing the circumstances of it, when he was at Venice in 1749, relinquished his former opinion, and concluded that it was occasioned either by the luminous aspect, or by some liquor or effluvia of an insect which he particularly describes, though he does not altogether exclude other causes, and especially the spawn or fry of fish.
The same hypothesis had also occurred to M. Vianelli; and both he and Grizellini, a physician in Venice, have given drawings of the insects from which they imagined this light to proceed.
A similar conjecture is proposed by a correspondent of Dr. Franklin, in a letter read at the Royal Society in 1756; the writer of which apprehends, that this appearance may be caused by a great number of little animals, floating on the surface of the Sea. And Mr. Forster, in his account of a voyage round the world with captain Cook, in the years 1772, 3, 4, and 5, describes this phenomenon as a kind of blaze of the Sea; and, having attentively examined some of the shining water, expresses his conviction that the appearance was occasioned by innumerable minute animals of a round shape, moving through the water in all directions, which show separately as so many luminous sparks when taken up on the hand: he imagines that these small gelatinous luminous specks may be the young fry of certain species of some medusæ, or blubber. And M. Dagelat and M. Rigaud observed several times, and in different parts of the ocean, such luminous appearances by vast masses of different animalcules; 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 animalcules.
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 liquors. M. le Roi is far from asserting that there are no luminous insects in the Sea; for he allows that several gentlemen have found them; but he is satisfied that the Sea is luminous chiefly on some other account, though he does not so much as offer a conjecture with respect to the true cause.
Other authors, equally dissatisfied with the hypothesis of luminous insects, for explaining the phenomenon which is the subject of this article, have ascribed it to some substance of the phosphoric kind, arising from putrefaction. The observations of F. Bourzes, above referred to, render it very probable, that the luminousness of the Sea arises from slimy and other putrescent matter, with which it abounds, though he does not mention the tendency to putrefaction, as a circumstance of any consequence to the appearance. But the experiments of Mr. Canton, which have the advantage of being easily made, seem to leave no room to doubt that the luminousness of the Sea is chiefly owing to putrefaction. And his experiments confirm an observation of Sir John Pringle's, that the quantity of salt contained in Sea-water hastens putrefaction; but since that precise quantity of salt which promotes putre- | faction the most, is less than that which is found in Sea-water, it is probable, Mr. Canton observes, that if the Sea were less salt, it would be more luminous. See Philos. Trans. vol. 59, pa. 446, and Franklin's Exper. and Observ. pa. 274.
What proportion the superficies of the Sea bears to that of the land, is not accurately known, though it is said to be somewhat more than two to one. This proportion of the surface of the Sea to the land, has been found by experiment thus: taking the printed paper map or covering of a terrestrial globe, with a pair of scissors clip out the parts that are land, and those that are water; then weighing these parcels separately in a pair of fine scales, the land is found to be near 1/3, and the water rather more than 2/3 of the whole.
With regard to the profundity or depth of the Sea, Varenius affirms, that it is in some places unfathomable, and in others very various, being in certain places from 1/20th of a mile to 4 1/2 miles in depth, in other places deeper, but much less in bays than in oceans. In general, the depths of the Sea bear a great analogy to the height of mountains on the land, so far as is hitherto discovered.
There are two special reasons why the Sea does not increase by means of rivers, &c, running every where into it. The first is, because waters return from the Sea by subterranean cavities and aqueducts, through various parts of the earth. Secondly, because the quantity of vapours raised from the Sea, and falling in rain upon the land, only cause a circulation of the water, but no increase of it. It has been found by experiment and calculation, that in a summer's day, there may be raised in vapours from the surface of the Mediterranean Sea, 528 millions of tuns of water; and yet this Sea receiveth not, from all its nine great rivers, above 183 millions of tuns per day, which is but about a third part of what is exhausted in vapours; and this defect in the supply by the rivers, may serve to account for the continual influx of a current by the mouth or straits at Gibraltar. Indeed it is rather probable, that the waters of the Sea suffer a continual slow decrease as to their quantity, by sinking always deeper into the earth, by filtering through the fissures in the strata and component parts.
