If you have ever been at the sea-side at a place where the tides are strong, and where the shore slants gently to the water, you must have remarked that every wave as it comes in runs several paces upward on the beach, and, immediately retreating, leaves uncovered again much of the surface which it had just overflowed, perhaps by many feet, until the next wave arriving plays the same part again. If the tide is rising you will observe that the retiring water does not fall quite to its former level, and that almost every fresh wave rolls a little higher up than did the one before it. The advance is scarcely perceived at first, but step by step it becomes more evident. About three hours after the moment of lowest ebb the rise of the tide is strongest; then again it becomes slower and slower, till, after three hours more, the flood-tide has reached its greatest height, at which it remains a short time without perceptible change. The fall of the tide begins as slowly as the flow, then the water retreats more quickly, and then again more slowly; and once more, after six hours, the ebb has reached its lowest; and there, again for a short time, its level is kept unchanged.

There are many places on the English shores where you may see the effect of these changes shown in striking contrasts of scenery; perhaps at none more beautifully than at Ilfracombe, in Devonshire. But at St. Malo, on the French coast, you may witness this wonderful phenomenon even on a grander scale.

At low tide St. Malo itself seems to be surrounded on three sides with wild craggy rocks, which are covered with mussels and sea-weed, and among which start up the lofty walls of the town. The level places between the cliffs are covered with a layer of fine sand, firm enough for walking, and formed almost entirely of powdered mussel-shells. Here and there are pools containing water, which its taste, as well as the number of little crabs, mussels, and star-fish, which may be found in it, show to be sea-water left behind

by the tide. A fringe of sea-weed marks upon the rocks the line which must be reached by the sea, whose roar is now only heard from a distance. And now, but a few hours later, how changed the scene! The town is almost

entirely surrounded by the sea, the waves of which are beating round the walls, breaking at their feet, and throwing the spray sometimes to their very top. The only communication with the land is now afforded by a long causeway, which you see at once to be the work of man, and which is no broader than the road which runs along it. On the side of this causeway towards the open sea, the rolling surge is striving against the barrier which meets it, dashing up in breakers thirty or forty feet high, and drenching with spray the wanderer who may tarry on the road.

The many cliffs which had been remarked before are now hidden under water, all but a few of the highest points of rocks, which you could have reached on foot before, but which now are islands in the sea. The other side of the mole is also washed by the sea. But here the fury of the waves is less, for it has been spent upon all the rocks and islets without; and as the flood has here run up far into the land-having had besides, after passing between the cliffs, to find its way around the town it retains but little of its former force. Here is the harbor of St. Malo, quite dry at low water, and at flood tide a great lake roomy enough for several thousand vessels, which, however, you will not see within it.

OCEAN CURRENTS.

PERPETUAL motion and change is the grand law to which the whole of the created universe is subject, and immutable stability is nowhere to be found but in the Eternal mind that rules and governs all things. The stars, which were supposed to be fixed to the canopy of heaven, are restless

wanderers through the illimitable regions of space. The hardest rocks melt away under the corroding influence of time, for the elements never cease gnawing at their surface, and dislocating the atoms of which they are composed. Our body appears to us unchanged since yesterday, and yet how many of the particles which formed its substance have, within these few short hours, been cast off and replaced by others. We fancy ourselves at rest, and yet a torrent of blood, propelled by an indefatigable heart, is constantly flowing through all our arteries and veins.

A similar external appearance of tranquillity might deceive the superficial observer, when sailing over the vast expanse of ocean, at a time when the winds are asleep, and its surface is unruffled by a wave. But how great would be his error! For every atom of the boundless sea is constantly moving and changing its place; from the depth to the surface, or from the surface to the depth; from the frozen pole to the burning equator, or from the torrid zone to the Arctic Ocean; now rising in the air in the form of invisible vapors, and then again descending upon our fields in fertilising showers.

The waters are, in fact, the greatest travellers on earth; they know all the secrets of the submarine world; climb the peaks of inaccessible mountains, shame the flight of the condor as he towers over the summits of the Andes, and penetrate deeper into the bowels of the earth than the miner has ever sunk his shaft.

Leaving their wanderings through the regions of air for the present, I shall now describe the principal ocean currents, the simple but powerful agencies by which they are set in motion, their importance in the economy of nature, and their influence on the climate of different countries.

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Even in the torrid zone, the waters of the ocean, like false friend, are warm merely on the surface, and of almost icy coldness at a considerable depth. This low tempera

ture cannot be owing to any refrigerating influence at the bottom of the sea, as the internal warmth of the earth increases in proportion to its depth, and the waters of profound lakes, in a southern climate, never show the same degree of cold as those of the vast ocean.

The phenomenon can thus only arise from a constant submarine current of cold water from the poles to the line, and, strange as it may seem, its primary cause is to be sought for in the warming rays of the sun, which, as we all know, distributes heat in a very unequal manner over the surface of the globe.

Heat expands all liquid bodies, and renders them lighter; cold increases their weight by condensation. In consequence of this physical law, the waters of the tropical seas, rendered buoyant by the heat of a vertical sun, must necessarily rise and spread over the surface of the ocean to the north and south, whilst colder and heavier streams from the higher latitudes flow towards the equator along the bottom of the ocean to replace them as they ascend.

In this manner, the unequal action of the sun calls forth a general and constant movement of the waters from the poles to the equator, and from the equator to the poles; and this perpetual migration is one of the chief causes by which their purity is maintained. These opposite currents would necessarily flow direct to the north or south, were they not deflected from their course by the rotation of the earth, which gradually gives them a westerly or easterly direction.

The unequal influence of the sun in different parts of the globe, and the rotation of the earth, are, however, not the only causes by which the course of ocean-currents is determined.

Violent storms move the waters to a considerable depth, and retard the flow of rivers, and thus it is to be expected that continuous winds, even of moderate strength, must have a tendency to impel the waters in the same direction,

The steady trade-winds of the tropical zone, and the prevailing westerly winds in higher latitudes, consequently unite their influence with that of the above-mentioned causes, in driving the waters of the tropical seas to the west, and those of the temperate zones to the east.

The tides also, which on the high seas generally move from east to west, promote the flow of the ocean in the same direction, and thus contribute to the westerly current of the tropical seas.

Nor must we forget that the obstacles which the oceancurrents meet on their way; such as intervening lines of coast, sand-banks, submarine ridges, or mountain chains, have a great influence upon their course, and may even give them a diametrically opposite direction to that which they would otherwise have followed.. Hartwig.

THE GULF STREAM.

THERE is a river in the ocean. In the severest droughts it never fails, and in the mightiest floods it never overflows. Its banks and its bottom are of cold water, while its current is of warm. The Gulf of Mexico is its fountain, and its mouth is in the Arctic seas. It is the Gulf Stream. There is in the world no other such majestic flow of waters. Its current is more rapid than the Mississippi or the Amazon.

Its waters, as far out from the Gulf as the Carolina coasts, are of an indigo blue. They are so distinctly marked, that their line of junction with the common seawater may be traced by the eye. Often one half of the vessel may be perceived floating in Gulf Stream water, while the other half is in common water of the sea; so sharp is the line, and such the want of affinity between those waters, and the reluctance, on the part of those of the Gulf Stream to mingle with the common water of the sea.