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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 forces--one perpendicular, and another tangent to the earth's surface. 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 north-west 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, whence, 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 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. 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
of the seas.
on each side of Australia, while the polar stream rushes along the Bay of Bengal; but the westerly current again becomes most powerful towards Ceylon and the Maldives, from whence it stretches by the extremity of the Indian peninsula, past Madagascar, to the most northern point of the continent of Africa, where it mingles with the general motion
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. 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 he found that the fields of ice were drifting to the south faster than his party could travel over them to the north.
Tas oscillations of the atmosphere, and the changes in its temperature, are measured by variations in the beights of the barometer and thermometer, but the actual length of the liquid columns in these instruments not only depends upon the force of gravitation, but upon capillary attractiou,, or the force of cohesion, which is a reciprocal attraction between the molecules of the liquid and those of the tube containing it.
All bodies consist of an assemblage of material particles held in equilibrio by a mutual affinity
or cohesive force which tends to unite them, and also by a repulsive force-probably caloric, the principle of heat—which tends to separate them. The intensity of these forces decreases rapidly, as the distance between the atoms augments, and becomes altogether insensible as soon as that distance has acquired a sensible magnitude. The particles of matter are so small, that nothing is known of their form further than the dissimilarity of their different sides in certain cases, which appears from their reciprocal attractions during crystallization being more or less powerful, according to the sides they present to one another. It is evident that the density of substances will depend upon the ratio which the opposing forces of cohesion and repulsion bear to one another.
When particles of the same kind of matter are at such distances from each other, that the cohesion which retains them is insensible, the repulsive principle remains unbalanced, and the particles have a tendency to fly from one another, as in aëriform fluids. If the particles approach sufficiently near to produce equilibrium between the attractive and repulsive forces, but not near enough to admit of any influence from their form, perfect mobility will exist among them, resulting from the similarity of their attractions, and they will offer great resistance when compressed, properties which characterize fluids, in which the repulsive principle
is greater than in the gases. When the distance between the particles is still less, solids are formed in consequence of the preponderating force of cohesion; but the nature of their structure will vary, because, at such small distances, the power of the mutual attraction of the particles will depend upon their form, and will be modified by the sides they present to one another during their aggregation.
All the phenomena of capillary attraction depend upon the cohesion of the particles of matter. If a glass tube of extremely fine bore, such as a small thermometer-tube, be plunged into a cup of water or alcohol, the liquid will immediately rise in the tube above the level of that in the cup, and the surface of the little column thus suspended will be concave. If the same tube be plunged into a cup full of mercury, the liquid will also rise in the tube, but it will never attain the level of that in the . сир,
and its surface will be convex. The elevation or depression of the same liquid in different tubes of the same matter is in the inverse ratio of their internal diameters, and altogether independent their thickness. Whence it follows that the molecular action is insensible at sensible distances, and that it is only the thinnest possible film of the interior surface of the tubes that exerts a sensible action on the liquid. So much indeed is this the case, that, when tubes of the same bore are com