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in Tonquin, where the tides arrive by two channels of lengths corresponding to half an interval, there is neither high nor low water on account of the interference of the

waves.

The initial state of the ocean has no influence on the tides; for, whatever its primitive conditions may have been, they must soon have vanished by the friction and mobility of the fluid. One of the most remarkable circumstances in the theory of the tides is the assurance that, in consequence of the density of the sea being only one-fifth of the mean density of the earth, and the earth itself increasing in density towards the centre, the stability of the equilibrium of the ocean never can be subverted by any physical cause. A general inundation arising from the mere instability of the ocean is therefore impossible. A variety of circumstances, however, tend to produce partial variations in the equilibrium of the seas, which is restored by means of currents. Winds and the periodical melting of the ice at the poles occasion temporary water-courses; but by far the most important causes are the centrifugal force induced by the velocity of the earth's rotation, and variations in the density of the sea. The centrifugal force may be resolved into two forcesone perpendicular, and another tangent to the earth's surface (N. 157). The tangential force, though small, is sufficient to make the fluid particles within the polar circles tend towards the equator, and the tendency is much increased by the immense evaporation in the equatorial regions from the heat of the sun, which disturbs the equilibrium of the ocean. To this may also be added the superior density of the waters near the poles, partly from their low temperature and partly from their gravitation being less diminished by the action of the sun and moon than that of the seas of lower latitudes. In consequence of the combination of all these circumstances, two great currents perpetually set from each pole towards the equator. But, as they come from latitudes

where the rotatory motion of the surface of the earth is very much less than it is between the tropics, on account of their inertia, they do not immediately acquire the velocity with which the solid part of the earth's surface is revolving at the equatorial regions; from whence it follows that, within twenty-five or thirty degrees on each side of the line, the ocean appears to have a general motion from east to west, which is much increased by the action of the trade winds. This mighty mass of rushing waters at about the tenth degree of south latitude is turned towards the northwest by the coast of America, runs through the Gulf of Mexico, and, passing the Straits of Florida at the rate of five miles an hour, forms the well-known current of the Gulf-stream, which sweeps along the whole coast of America and runs northward as far as the bank of Newfoundland, then bending to the east it flows past the Azores and Canary Islands, till it joins the great westerly current of the tropics about latitude 21° north. According to M. de Humboldt, this great circuit of 3800 leagues, which the waters of the Atlantic are perpetually describing between the parallels of eleven and forty-three degrees of latitude, may be accomplished by any one particle in two years and ten months. In the centre of this current is situated the wide field of floating sea-weed called the grassy sea. Besides this there are branches of the Gulf-stream, which convey the fruits, seeds, and a portion of the warmth of the tropical climates to our northern shores.

The general westward motion of the South Sea, together with the south polar current, produce various water-courses in the Pacific and Indian Oceans, according as the one or the other prevails. The western set of the Pacific causes currents to pass on each side of Australia, while the polar stream rushes along the Bay of Bengal: the westerly current again becomes most powerful towards Ceylon and the Maldives, whence it stretches by the extremity of the Indian

peninsula past Madagascar to the most southern point of the continent of Africa, where it mingles with the general motion of the seas. Icebergs are sometimes drifted as far as the Azores from the north pole, and from the south pole they have come even to the Cape of Good Hope. But the ice which encircles the south pole extends to lower latitudes by 10° than that which surrounds the north. In consequence of the polar current Sir Edward Parry was obliged to give up his attempt to reach the north pole in the year 1827, because the fields of ice were drifting to the south faster than his party could travel over them to the north.

The theory of under-currents setting from the poles to the equator is proved to be erroneous by Kotzebue and Sir James Ross, who found a stratum of constant temperature in the ocean at a depth depending upon the latitude: at the equator it is at the depth of 7200 feet, from whence it gradually rises till it comes to the surface in both hemispheres about the latitude of 56° 26′, where the water has the same temperature at all depths; it then descends to 4500 feet below the surface about the 70th parallel both in the Arctic and Antarctic Seas. The temperature of that aqueous zone is about 390.5 of Fahrenheit.

I

SECTION XIV.

Repulsive Force-Interstices or Pores-Elasticity-Mossotti's Theory-Gravitation brought under the same Law with Molecular Attraction and Repulsion-Gases reduced to Liquids by Pressure-Intensity of the Cohesive Force-Effects of Gravitation-Effects of Cohesion-Minuteness of the ultimate Atoms of Matter-Limited Height of the Atmosphere-Theory of Definite Proportions and Relative Weight of Atoms-Dr. Faraday's Discoveries with regard to Affinity-Composition of Water by a Plate of Platina -Crystallisation-Cleavage-Isomorphism-Matter consists of Atoms of definite Form-Capillary Attraction.

THE Oscillations of the atmosphere, and its action upon rays of light coming from the heavenly bodies, connect the science of astronomy with the equilibrium and movements of fluids, and the laws of molecular attraction. Hitherto that force has been under consideration which acts upon masses of matter at sensible distances; but now the effects of such forces are to be considered as act at inappreciable distances upon the ultimate atoms of material bodies.

All substances consist of an assemblage of material particles, which are far too small to be visible by any means human ingenuity has yet been able to devise, and which are much beyond the limits of our perceptions. Since every known substance may be reduced in bulk by pressure, it follows that the particles of matter are not in actual contact, but are separated by interstices, owing to the repulsive principle that maintains them at extremely minute distances from one another. It is evident that, the smaller the interstitial spaces, the greater the density. These spaces appear in some cases to be filled with air, as may be inferred from certain semi-opaque minerals and other substances becoming transparent when plunged into water; sometimes they may possibly contain some unknown and highly elastic fluid, such

as Sir David Brewster has discovered in the minute cavities of various minerals, which occasionally causes these substances to explode with violence when under the hands of the lapidary, but in general they seem to our senses to be void; yet, as it is inconceivable that the particles of matter should act upon one another without some means of communication, there is every reason to presume that the interstices of material substances contain a portion of that subtle ethereal and elastic fluid with which the regions of space are replete.

Substances compressed by a sufficient force are said to be more or less elastic according to the facility with which they regain their bulk or volume when the pressure is removed; a property which depends upon the repulsive force of their particles; and the effort required to compress the substance is a measure of the intensity of that repulsive force which varies with the nature of the substance.

By the laws of gravitation the particles of matter attract one another when separated by sensible distances; and, as they repel each other when they are inappreciably near, it recently occurred to Professor Mossotti, of Pisa, that there might be some intermediate distance at which the particles might neither attract nor repel one another, but remain balanced in that stable equilibrium which they are found to maintain in every material substance solid and fluid.

It has long been a hypothesis among philosophers that electricity is the agent which binds the particles of matter together. We are totally ignorant of the nature of electricity, but it is generally supposed to be an ethereal fluid. in the highest state of elasticity surrounding every particle of matter; and, as the earth and the atmosphere are replete with it in a latent state, there is every reason to believe that it is unbounded, filling the regions of space.

The celebrated Franklin was the first who explained the phenomena of electricity in repose, by supposing the molecules of bodies to be surrounded by an atmosphere of the

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