This section is from the book "A Library Of Wonders And Curiosities Found In Nature And Art, Science And Literature", by I. Platt. Also available from Amazon: A library of wonders and curiosities.
The sea, or ocean, is that vast tract of water which encompasses the whole earth. What proportion the superficies of the sea bears to that of the land, is not precisely known, though it is said to be somewhat more than two-thirds. As the waters of the earth must necessarily rise to the surface thereof, it being specifically lighter than the earth, it was necessary there should be large cavities therein, as receptacles to contain them, otherwise they would have overspread all the surface of the earth, and so have rendered it utterly uninhabitable for terrestrial animals: it is well known, that the centre of the earth is the common centre of gravity, and that the nature of fluids is such, that they equally yield to equal powers; hence it follows, that where the power of attraction is every where the same at equal distances from the centre, the superficial parts of the water will every where conform themselves to this attractive power, at an equidistant situation from the centre, and, it is evident, will form the surface of a sphere, so far as they extend. The reason then that the sea seems higher than the land, results from the fallacy of vision, whereby all objects, whether on the land or sea, appear higher as they become more distant: and the reason will be plain to those who are acquainted with optics; for it is well known, that the denser any medium is, through which we behold objects, the greater is the refraction, or the more their images appear above the horizontal level; while the greater the quantity of medium through which the rays pass, the more they will be bent from their first direction: on both these accounts, the appearances of things at a great distance, both on the land and the sea, will be somewhat above the horizon, and the more so as they are the more remote.
With regard to the depth or profundity of the sea, Vare-riius affirms, that it is in some places unfathomable, in other places very various, being from fifty yards to four and a half English miles, in some places deeper, and that the depth is 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 discoveries have hitherto extended. 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 waters. Thus, the deepest part of the Mediterranean is generally allowed to be under the heights 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 the bottom of the sea; for the veins of salt and bitumen doubtless run on in the same order as we see them on the shore. If we may reason from analogy, the strata of rocks, that serve as a foundation for hills and elevated places on shore, serve also, in the same continued chain, to support the immense quantity of water in the basin 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 materials, such as could not be imagined by any but those who have examined into them, 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 is the first part that touches the ground; and the soft nature of the fat receives into it some part of those substances which it meets with at the bottom: the substances thus brought up, are sometimes pure sand, sometimes a kind of sand made of the fragments of shells beaten to a sort of powder, sometimes they are composed of a like powder to the several sorts of corals, and sometimes they are composed of fragments of rocks; but besides these appearances, which are natural enough, and are what might well be expected, it brings up substances which are of the most beautiful colours.
Dr. Donati, in an Italian work, containing an essay on a natural history of the Adriatic Sea, has related many curious observations on this subject: 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 was very little difference between the former and the latter. At the bottom of the water there are mountains, plains, valleys, 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 petrified bodies, pumice stones, and lavas formed by volcanoes. One of the objects which most excited his attention, was a crust, which he discovered under the water, composed of crustaceous and testaceous bodies, with 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 the natural beds of the sea, and not made so by means of volcanoes 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.
M. Dassie has been at great pains to prove, that the sea has a genera] motion, independently of winds and tides, and that it is of more consequence in navigation than is generally supposed. He affirms, that this motion is from east to west; inclining towards the north, when the sun has passed the equinoctial northward, during the time he is passing through the northern signs; but the contrary way, after the sun has passed the said equinoctial southward: adding, that when this general motion is changed, the diurnal flux is changed also; whence it happens, that in several places the tides come in during one part of the year, and go out during the other, as on the coasts of Norway, in the Indies at Goa, Cochin-china, etc. where, while the sun is in the summer signs, the sea runs to the shore; and when in the winter signs, runs from it. On the most southern coasts of Tonquin and China, for the six summer months, the diurnal course runs from the north with the ocean; but the sun having repassed the line toward the south, the course declines also southward.
There are two principal reasons why the sea does not increase by means of rivers, etc. falling 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 on the land, only cause a circulation, but no increase of water. It has been found, by calculation, that in a summer's day there may be raised in vapours, from the Mediterranean Sea, 5,280,000,000 tons of water, and yet this sea receiveth not, from all its nine great rivers, above 1,827,000,000 tons per day, which is but a third part of what is exhausted in vapours.
The ascent of the sea for the formation of springs, by a subterranean circulation of its water to their sources, has been a great objection, with many, against the system which ascribes their origin to the ocean; but Dr. Plot has observed, that there are many ways by which the water may ascend above its own level: 1. By the means of subterranean heat. 2. By nitration. 3. By the unequal height of several seas. 4. By the distance of the centre of magnitude from the centre of gravity in the terraqueous globe; the superficies of the Pacific Sea being said to be further from the centre of gravity than the top of the highest hill on the adverse part of the globe. And, 5. By the help of storms. The sea water actually ascends above its own level, and finds its way into wells, whose bottoms lie higher than the surface of the sea at high-water mark.