is only 38448000 of gravitation at the earth's surface, and the action of the moon is little more than twice as much; these forces being in the ratio of 1 to 2:35333, when the sun and moon are at their mean distances from the earth. From this ratio, the mass of the moon is found to be only 1 of that of the earth. Had the action of the sun on the ocean been exactly equal to that of the moon, there would have been no neap tides, and the spring tides would have been of twice the height which the action of either the sun or moon would have produced separately; a phenomenon depending upon the interference of the undulations.

A stone plunged into a pool of still water occasions a series of waves to advance along the surface, though the water itself is not carried forward, but only rises into heights and sinks into hollows, each portion of the surface being elevated and depressed in its turn. Another stone of the same size, thrown into the water near the first, will occasion a similar set of undulations. Then, if an equal and similar wave from each stone arrive at the same spot at the same time, so that the elevation of the one exactly coincides with the elevation of the other, their united effect will produce a wave twice the size of either; but if one wave precede the other by exactly half an undulation, the elevation of the one will coincide with the hollow of the other, and the hollow of the one with the elevation of the other, and the waves will so entirely obliterate one another, that the surface of the water will remain smooth and level. Hence, if the length of each wave be represented by 1, they will destroy one another at intervals of 1, 2, §, &c. and will combine their effects at the intervals 1, 2, 3, &c. It will be found, according to this principle, when still water is disturbed by the fall of two equal stones, that there

3 5

21

are certain lines on its surface of a hyperbolic form, where the water is smooth in consequence of the waves obliterating each other; and that the elevation of the water in the adjacent parts corresponds to both the waves united. Now, in the spring and neap tides, arising from the combination of the simple soli-lunat waves, the spring tide is the joint result of the combination when they coincide in time and place; and the neap tide happens when they succeed each other by half an interval, so as to leave only the effect of their difference sensible. It is therefore evident that, if the solar and lunar tides were of the same height, there would be no difference, consequently no neap tides, and the spring tides would be twice as high as either separately. In the port of Batsha, 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 onefifth of the mean density of the earth, and that the earth itself increases in density toward the centre, the stability of the equilibrium of the ocean never can be subverted by any physical cause whatever. 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 tempo

rary water-courses; but by far the most important causes are the centrifugal foree 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. 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 wellknown 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 21o 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, produces various watercourses 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; 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 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. 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.

SECTION XV.

The oscillations of the atmosphere, and the changes in its temperature, are measured by variations in the heights 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 attraction, 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

232313