The coral fisheries have given occasion to observe that there are many, and those very large, caverns or hollows in the bot. torn of the sea, especially where it is rooky ; and that the like caverns are some times found in the perpendicular rocks which form the steep sides of those fishe ries. These caverns are often of great depth, as well as extent, and have some time wide mouths, and sometimes only narrow entrances into large and spacious The bottom of the sea is covered with a variety of matters, such as could not be imagined by any but those who have ex amined Into it, especially in deep water, where the surface only is disturbed by tides and storms, the lower part, and con sequently its bed at the bottom, remain ing for ages, perhaps, undisturbed The soundings, when the phimmit first touch es the ground, on approaching the shores, give some idea of this. The bottom of the plummit is hollowed, and in that hol low.there is placed a lump of tallow ; whip being the part that first touches the ground, the soft nature of the fat receives into it some parts of those sub stances which it meets with at the bottom : this matter, thus brought up, is sonie. timets pure sand; sometimes a kind of sand made of the fragment of shells, bea ten to a sort of powder ; sometimes it is made of a like powder of 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.
Dr. Donati, in an Italian work, contain ing an essay towards a natural history of the Adriatic Sea, has related many curi ous observations on this subject; having carefully examined the soil and produc tions of the various countries that sur round 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 for mer and the latter. At the bottom of the water there are mountains, plains, willies and caverns, similar to those upon land. The soil consists of different stra ta, placed one upon anoiher, and mostly parallel and correspondent to those of the rocks, islands, and neighbouring conti nents. 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 dis covered under the water, composed of crustaceous and testaceous bodies, and beds of poly pes of different kinds, confu sedly blended with earth, sand, and gra vel ; the different marine bodies which form this crust are found at the depth of a foot or more, entirely petrified, and re duced into marble ; these, he suppoies, are naturally placed under the sea when it covers them, and nut by means of vol canoes 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 re cites a great number of facts.
M. Dassie has been at great pains to prove, that the sea has a general motion, independently of winds and tides, and of more consequence in navigation than is generally supposed. He affirms, that this motion is from east to west, inclining to• wards the north, when the sun has passed the equinoct ial northward, and that during the time the sun is in 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, &c. 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 moat southern coasts of Tonquin and China, fbr the six summer months, the diurnal course runs from the north with the ocean ; but the sun having re-passed the line toward the south, the course declines also south ward.
There are two principal reasons why the sea does not increase by means of ri vers, &c. falling every where into it. the first is, because waters return from the sea by subterranean cavities and aque ducts, through various parts of the earth. Secondly, because the quantity of va pours 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 Medi terranean Sea, 5,280,000,000 tons of wa ter ; 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 va pours.
The ascent of the sea-water, 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 of their being formed of the sea; 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 heats.
2. By filtration. 3. By the unequal height of several seas. 4. By the distituce of the centre of magnitude from the centre of gravity, in the terraqueous globe : the superficies of the Pacific Sea is 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 ac tually ascends above its own level, com ing into wells whose bottoms lie higher than the surface of the sea at high-water mark.