that the waters at A, the part of the earth nearest the moon, will most feel the effect of her attraction, and will be raised up to B; while those on each side of A, being farther off, will be less raised. But besides the difference occasioned by a greater or less distance, the more oblique or slanting the line of attraction, the less will be the elevation of the water acted upon, till at last the water towards and under the circle CEF will not only not be elevated but will be lowered. The reason of this is, that the force of attraction acts in straight lines; and, therefore, if we draw two straight lines from the moon's centre, MC, MF, to represent this force acting on the parts C and F, it is obvious that the water at C and at F will not be raised, but depressed by being drawn away from C to D and from F to H; and so of every part of the circle CEF. In the same manner, the water at D and H will pass to G and I, and thus the ocean will be disposed in a spheroidal form CDGBIHF. But the water will, at the same time, rise on the side of the earth away from the moon, because the earth's centre being more strongly drawn towards the moon than the point N, recedes from N, which is the same in effect as if the water at N receded or rose up from the earth's centre. The ocean, therefore, will assume a spheroidal form, CKF, on the side away from the moon, as well as on the one facing her. Thus, if we draw a line MAK from the moon's centre through the centre of the earth, the two points B, K, where it touches the earth's surface, will be those of high water; and if we take two more points C, F, equally distant from each of the first two, they will be points of low water. By the earth's rotation on its axis the part F will be carried to A and the part C to N; matters will then be just reversed, and it will be high water at F

and C, and low water at A and N. According to th is it should be high water at any place in the open sea, when the moon is upon the meridian of that place, and low water when the moon is upon a circle cutting the meridian in question at right angles; but, in fact, the greatest and least heights of the water at such a place do not occur till about three hours after the periods fixed in this supposition. The delay is thus explained: the elevated parts of the sea have received such an impulse towards ascent, that they continue to rise after the earth's rotation has carried them from under the line of the direct attraction of the moon; this impulse being also aided for a time by the moon continuing to attract the water upwards, though in a less degree.

As the moon crosses the meridian of a place about every twenty-four hours fifty minutes and a half, the sea in that space of time ebbs twice and flows twice all over the world, although much less towards the poles than within the tropics, where the waters are under the direct line of the lunar attraction.

In the above remarks we have spoken only of the moon, because, though the sun is so very much larger than the moon, yet the latter, on account of her nearness to our planet, has the most powerful effect upon the tides; it is calculated that her influence is nearly triple that of the sun. The sun, however, acts upon the ocean in the same manner, though in a less degree. When these two bodies unite their influence, which they do at the seasons of new and full moon, the tides naturally rise the highest, and are then called spring tides; but when the moon is in her quadratures, or quarters, the action of each of the two luminaries is directly opposed to that of the other; the tides are then of course the lowest, and are called neap

tides. To explain this more clearly, let E (fig. 2) be the earth, S the sun, and m the moon; when new, the moon is situated at x, when full at x', and when in her quadratures at y and y'. It results, from what has been already said, that, both at new and full moon, the sun and moon will each assist the other in raising the ocean round A and D, and depressing it at C and B; but when the moon is in her quadratures, her tendency is to raise the waters at C and B and depress them at A and D; while that of the sun is exactly the reverse. During the moon's circuit round the earth, the

spring and the neap tides each occur twice, and one after the other.

If the earth were entirely covered by a sea of uniform depth, and the sun and moon moved always in the plane of the equator, the region of the highest tides would always be directly under the equator, while at the poles there would never be any tide whatever. But the changes that occur in the positions of the sun and moon, and several other circumstances, prevent the tides from taking place in so uniform a manner.

In figure 3, let us suppose AEH to be the equator, and C and F the poles

water at the poles, that is, they would never have any tides at all. The sun and moon, however, are continually changing their positions, with respect both to the equator and to each other, and corresponding variations are produced in the tides. The moon, for instance, is sometimes as much as 283 degrees on each side of it. Suppose her to be situated at x, 28 degrees north of the equator, draw a line from a through the centre of the earth, and let it come out at the opposite side of the earth, the points B and I will have the highest tides, and as the earth turns round, all the parts of the circles G B and I D will successively come to B and I. The waters, under those two circles, will,

of the earth, and let M, the moon, and S, the sun, both be in the plane of the equator; in this case the water will be higher at H and A, than in any other places, and most depressed under the circle CEF. By the earth's rotation, E will come to A, and it will be high water at E, but C and F, the poles, suffer no change of position from the earth's rotation, and the waters there will, consequently, remain just as before. If the sun and moon, therefore, were to remain constantly in the plane of the equator, the highest tideswould always be at A, E, H, &c., that is, under the equator; while, as Cand F would always be situated the same with respect to the sun and moon, it would always be low

of the manner in which tides are created in the ocean, will enable us to perceive why it is that, in some gulfs and inland seas, there are either no tides, or such trifling ones as to be scarcely discernible. In figure 1, the waters at A are brought to B, not only by the moon raising up the parts immediately under her, but also by her drawing obliquely towards B the parts distant from B, and by the lateral flowing of the neighbouring waters (p n, for instance) towards B, which results from their being less attracted by the moon, and, therefore, heavier than those at B. Being heavier than B, they press upon and flow towards that part. In small collections of water, the moon acts with the same line of attraction, or nearly so, upon every portion of the surface at once, and, therefore, the whole of the waters being equally elevated at the same period, no part of them is ever higher than the other. This is one reason why the Baltic has no perceptible tides, and why even those of the Mediterranean are hardly visible.* But in addition to this, the two seas in question are so circumstanced that they cannot receive tides from the Atlantic: 1st, because their entrances are not turned towards the main direction of the Atlantic tide; 2ndly, because their entrances are so narrow, that the quantity of tide which that ocean can, in a few hours, impel into them, is insufficient, after being spread over the extensive surfaces of the two seas, to raise their level at all perceptibly. The Greeks, who accompanied Alexander the Great in his expedition to the east, having never been on any other coasts than those of the Mediterranean, were seized with complete consternation on first beholding the retreat of the strong tide which the Indian ocean sends into the river Indus. In gulfs which are differently circumstanced with respect to the direction of their entrances, and which have openings wider, as compared with their extent, the tides propagated from the ocean are sensibly felt. Hudson's and Baffin's Bays, and the Red Sea, are examples which prove the correctness of this observation.

The little tide which there is in the Mediterranean, seems to be formed chiefly in the part extending to the east of Malta, and to proceed northward into the Gulf of Venice. M. D'Angos observed that at Toulon, on the coast of France, the sea rose a foot about three hours and a half after the moon passed the meridian.

Currents and winds (especially the latter) have, according to their direction, an influence either in quickening or retarding the tide; indeed a powerful wind will sometimes keep a tide out of very narrow channels. On the contrary, a strong wind coming from the same quarter as the tide, will raise it several feet above its usual level.

The causes which render the movements of the tides complex and irregular, may thus be summed up under four heads-1. The variations in the positions of the sun and moon, with respect to the equator and to each other. 2. The obstacles presented by the land; 3. by winds; and 4. by currents. The existence of these causes renders it impossible to lay down any general rule for calculating the level, either of high or of low water, in different latitudes.

3. Currents in the ocean may be occasioned in various ways: they may arise from an external impulse, (a gale of wind for instance); from a difference in temperature or saltness between two parts of the sea; from the periodical melting of the polar ice, or from the inequality of the evaporation which the surface of the sea undergoes in different latitudes. These causes may produce either constant or occasional currents, and, according as they act in concert or in opposition, will their effects be various.

The most remarkable currents are those which continually follow the same direction. There is one which sets regularly from each of the poles towards the equator; and when we get within twenty-eight or thirty degrees of the line on either side, a general movement is observed in the ocean, in a direction nearly from east to west. The existence of the two polar currents is proved by the floating of masses of ice from the frigid into the temperate regions: these masses are, at times, seen as low as the forty-fifth, or even the fortieth degree of latitude. It was the opposition of the polar current which principally occasioned the failure of the attempt made last year under Captain Parry to reach the north pole; before they desisted from their efforts, the expedition found that, as they advanced over the ice, they were being drifted southward, at a rate faster than that at which they were travelling northward. It is equally certain that a tropical current exists, judging not only from the direction of bodies floating on the water, but also from the circumstance that vessels, in crossing from

Europe to America, descend to the latitude of the Canary Islands, where they fall into a current and are carried rapidly to the west. In going from America to Asia across the Pacific, a similar effect is observed. It might be supposed that this was due solely to the trade-winds, but such is not the case: for it is quite possible to distinguish their effect from that of the currents, since the progress of the vessel is quicker than it could be with the aid of the wind alone. The origin of the polar currents is, no doubt, in a great measure, to be referred to the centrifugal force which is the result of the earth's rotation. (See Mathematical Geography, chap. 8.) It may be further explained, when we reflect that the water towards the poles, both on account of its lower temperature and its being less attracted by the heavenly bodies, is heavier than the water in the tropical regions, and, moreover, that the heat of the torrid zone occasions a much more powerful evaporation of the sea than is elsewhere experienced: the consequence is, that the waters nearer the poles will move towards the equator, in order to restore the equilibrium which has, in these several ways, been destroyed. The tropical current may also, though in another manner, be explained as proceeding from the earth's rotation. The waters, as they advance from the polar seas, pass from regions where the rotatory motion of the earth's surface is very slight, to those where it is exceedingly rapid; they cannot immediately acquire the rapid motion with which the solid parts of the earth revolve in the tropical regions, and they are, accordingly, left rather behind, that is, to the westward (the earth turning round from west to east). The ocean, consequently, appears to retreat from the western, and advance upon the eastern coasts of the continents, or, in other words, to have a general movement from east to west; and the effect is very much assisted by the constant blowing of the trade winds.

We will now explain the modifications or changes which this grand movement in the ocean undergoes, in consequence of the obstacles presented by the land to its free progress. When it meets with shores or narrow straits to impede or turn aside its course, it forms strong and even dangerous currents. The eastern coast of America, and the West India Islands, constitute a sort of dyke to the general westward motion of the

Atlantic; and it will be seen, if we refer to a map, that from Cape St. Roche, which has about five degrees of south latitude, the coast of South America stretches away in a continued line to the north-west, as far as the isle of Trinidad. Owing to this shape of the coast, the waters, as far as the tenth degree of south latitude, are, when they approach America, carried away in a current to the north-west. This current afterwards enters the gulf of Mexico, through the strait formed by the western end of Cuba, and the opposite peninsula, (from this part it is called, by navigators, the Gulf-stream,) and follows the bendings of the Mexican coast, from Vera Cruz to the mouth of the Rio del Norte, and thence to the mouths of the Mississippi, and the shoals west of the southern extremity of Florida. It next takes a new direction to the north, and rushes impetuously into the gulf of Florida. M. Humholdt observed in the month of May 1804, in the 26th and 27th degrees of latitude, that its velocity was eighty miles in twenty-four hours, although, at the time, there was a violent wind against it. At the end of the gulf of Florida, (north lat. 28°) it runs to the north-east, at the rate, sometimes, of five miles an hour. It may always be distinguished by the high temperature * and the saltness of its waters, their indigo-blue colour, and the quantity of sea-weed floating on the surface, and also by the heat of the surrounding atmosphere. The rapidity and temperature of the Gulf-stream, diminish towards the north, while, at the same time, its breadth increases. Its further progress northward is at last checked by the southern extremity of the great bank of Newfoundland, in the 42d degree of latitude, where it turns suddenly to the east. It afterwards continues moving towards the east, and the east-southeast, as far as the Azores islands; and

Humboldt observes that "the waters of the Mexican Gulf, forcibly drawn to the north-east, preserve their warm temperature to such a point, that at forty and forty-one degrees of latitude, he found them at seventy-two degrees and a half (Fahrenheit); when, out of the current, the heat of the ocean at its surface was scarcely sixty-three degrees and a half. In the parallel of New York, (forty-one degrees north) the equal to that of the seas of the tropics in the eighteenth temperature of the Gulf stream is, consequently, degree of latitude."

Its breadth in latitude twenty-eight degrees and a half is seventeen leagues; (3.46 miles to a league) in the parallel of Charles town, (thirty-three degrees. nearly from forty to fifty leagues; and on the meri

dian of Corvo and Flores, the westernmost of the Azores islands, it is one hundred and sixty leagues.