but as the sun and moon have a like principle of gravitation towards their centres, and the earth is within the activity of their attraction, it plainly follows, that the equality of the pressure of gravity on the earth must be disturbed, and the ocean, be. ing fluid and yielding to the least force, shows where it is pressed according to its rising or sinking. Now the moon being very near the earh, in comparison of the sun and other heavenly bo❤ dies, our tides are chiefly regulated by its motions; and accordingly we observe, that the sea flows as often as the moon cuts the meridian, whether above or below the horizon; and ebbs when she passes the horizon, both in the eastern and western point: but as the moon is twelve hours and twenty-four minutes in passing from the superior to the inferior meridian, so every tide of flood is twenty-four mi nutes later than the preceding one; and thus we have two fluxes and two refluxes every five and twenty hours.

The high spring tides, upon the new and full moons, are occasioned by the attraction of the sun conspiring with that of the moon, whereas in the quarters the tides are weaker, because the sun raises the water where the moon depresses it. The reason why the sun's attraction has no greater in fluence on the tides, notwithstanding he is ten thousand times bigger than the earth and moon, is owing to the very small proportion the semidiame ter of the earth bears to his immense distance. It is also observed, that the equinoctial spring-tides in March and September are the highest, and the nep-tides the lowest of all others; for the nearer the moon approaches the poles, the less is the agitation of the ocean, which is greatest of all when the moon is in the equinoctial, or farthest

distant from the poles; whence the sun and moon being either conjoined or opposite in the equinoctial, produce the greatest spring-tides, and the subsequent nep-tides are always the least, being produced by the tropical moon in the quarters.

But besides these general tides, which would happen regularly every where, if the earth were all covered with deep sea, there are many others in which we find a vast diversity, and not to be accounted for, without an exact knowledge of local circumstances, as the position of the land, the shallowness of the water, the narrowness of the channels, &c. for the tide is always found to set strongest where the sea is narrowest, the same quantity of water being in that case to run through a smaller passage. This is evident between Portland and Cape la Hogue in Normandy, where the tide runs like a sluice, and would be yet stronger between Dover and Calais, if it were not checked by the tide coming round the island. In short, every thing relating to the tides is to be accounted for from the Newtonian doctrine; as why lakes, such as the Caspian sea, and midland seas, such as the Baltic, the Black Sea, and the Mediterranean, have scarcely any sensible tides; for lakes, having no communication with the ocean, can nei ther increase nor diminish their water, so as to rise or fall; and seas that communicate with it by such narrow inlets, and are of such a vast extent, cannot in a few hours receive or empty water enough to raise or sink their surface in a sensible manner. Sir Isaac Newton accounts for the strong tides in the port of Tonquin in China, (where there is but one flood and ebb in twenty-four hours, and none at all when the moon is near the equinoctial) from the concurrence of two tides, the one out of

the great South Sea, the other out of the Indian Sea between the islands; and as the appearance of these tides is naturally deducible from his princi ples, it is a strong argument in favour of his whole theory. ༨༣༠༥19b

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We shall conclude this subject, with observing, that though the Mediterraneau has no sensible tides, except some small ones in the gulf of Ve nice, that of the Euripus abovementioned, c yet a strong current continually sets into it from the ocean through the straits of Gibraltar, and likewise through the Hellespont from the Euxine and the Propontis; whence one would imagine, that instead of not swelling like the ocean, it should rather overflow, its bounds, and inundate the adjacent countries. What becomes of the vast quantity of water, thus poured into the Mediterranean, is a speculation that has long employed the philosophers. Dr. Smith accounts for it, by supposing an under current to carry off as much water as the upper one brings in; and such currents it is probable there are in several parts of the sea but Dr. Halley, without having recourse to this hypothesis, solves the phenomenon from the great evaporation. The result of an experiment, made by this excellent author, to find the quantity of vapour raised from the sea by the action of the sun, was, that the thickness of water evaporated from the surface of the sea in summer, is one fifty-third part of an inch in the space of two hours; which, for the case of calcu lation, being supposed only a sixtieth part, the quantity exhaled in twelve hours will be one tenth of an inch. On this principle every square mile will be found to evaporate, in twelve hours, six thousand nine hundred and fourteen tuns of water,

and every square degree, supposed of sixty-nine English miles, will evaporate thirty-three millions of tuns. Now the area of the Mediterranean being estimated at one hundred and sixty square degrees, it will lose in vapour, in a summer's day, five thousand two hundred and eighty millions of tuns; and yet this quantity of vapour, great as it is, is only the remains of what is raised by the winds, which sometimes sweep off the water faster than it is exhaled by the heat of the sun.

With respect to the quantity of water received by the Mediterranean, the Doctor supposes the Ebre, Rhone, Tyber, Po, Danube, Niester, Boristhenes, Tanais, and Nile, to furnish each of them ten times as much water as the Thames; not that any of them are in reality so great, but so to allow for the lesser rivers that fall into it; and as the Thames is computed to evacuate daily twenty million three hundred thousand tuns of water, the nine rivers abovementioned will only evacuate one thousand eight hundred and twentyseven millions of tuns in a day, which is little more than a third of what is raised in that time in vapour. To this vast store of vapours raised by the sun, winds, or subterraneous fires, from the sea, lakes, rivers, &c. the Doctor refers the origin of springs.

This account may serve to explain why the Caspian sea, into which many vast rivers discharge themselves, and which has no visible outlet, does 'not overflow its banks; and it may also show why the Almighty has placed spacious lakes in many other countries, at a distance from the sea. Were it not for these bodies of water that supply the clouds with rain, such countries would have no refreshing showers; the reservoirs in the hills and

mountains, which gush out in springs, and give rise to many rivers, would soon fail; water would be in a manner unknown; vegetation would gra-. dually cease; and many regions of great extent, now remarkable for their fertility, would become barren and desolate.

MINERALS, FOSSILS, &c.

THE Turkish dominions in Europe being very extensive, and the soil various, afford a great quantity of useful fossils, and some mines of the richer metals. Near Siderocapsa, a towh of Macedonia, lying north-east of Solonichi, mines of gold were discovered in the time of Philip, the father of Alexander the Great, from whence the town was anciently called Chrysitis.-Argentiera, a little island in the Archipelago, derives its modern name from its mines of silver, which were once wrought; and M. Tournefort informs us, that the workhouses and furnaces, where they used to prepare the metal, are still to be seen; but at present the inhabitants dare not meddle with that sort of work without asking permission of their despotic masters, who would make it a pretence for loading them with heavy taxes.-Siphanto, another island of the Archipelago, was anciently famous for its mines of gold and silver; but the places where they were, are now scarcely known to the inhabitants. The island, however, abounds with lead, according to M. Tournefort, who tells us, that about the middle of the last century, some Jews came thither, by order of the Porte, to ex