ther it consists of particles infinitely divisible or not. On the latter hypothesis, it must really be finite, and even if its particles be infinitely divisible, it is known, by experience, to be of extreme tenuity at very small heights. The barometer rises in proportion to the superincumbent pressure. At the level of the sea, in the latitude of 45°, and at the temperature of melting ice, the mean height of the barometer being 29-922 inches, the density of air is to the density of a similar volume of mercury, as 1 to 10477-9, consequently the height of the atmosphere, supposed to be of uniform density, would be about 4-95 miles; but as the density decreases upwards in geometrical progression, it is considerably higher, probably about fifty miles. The air, even on the mountain-tops, is sufficiently rare to diminish the intensity of sound, to affect respiration, and to occasion a loss of muscular strength. The blood burst from the lips and ears of M. de Humboldt as he ascended the Andes, and he experienced the same difficulty in kindling and maintaining a fire at great heights that Marco Polo, the Venetian, did on the mountains of Central Asia. At the height of thirty-seven miles, the atmosphere is still dense enough to reflect the rays of the sun when eighteen degrees below the horizon; and although at the height of fifty miles, the bursting of the meteor of 1783 was heard on earth like the

report of a cannon, it only proves the immensity of the explosion of a mass, half a mile in diameter, which could produce a sound capable of penetrating air three thousand times more rare than that we breathe; but even these heights are extremely small when compared with the radius of the earth.

The action of the sun and moon disturbs the equilibrium of the atmosphere, producing oscillations similar to those in the ocean, which ought to occasion periodic variations in the heights of the barometer. These, however, are so extremely small, that their existence in latitudes far removed from the equator is doubtful. M. Arago has lately been even led to conclude that the barometrical variations corresponding to the phases of the moon are the effects of some special cause, totally different from attraction, of which the nature and mode of action are unknown. La Place seems to think that the flux and reflux distinguishable at Paris may be occasioned by the rise and fall of the ocean, which forms a variable base to so great a portion of the atmosphere.

The attraction of the sun and moon has no sensible effect on the trade winds; the heat of the sun occasions these aërial currents, by rarefying the air at the equator, which causes the cooler and more dense part of the atmosphere to rush along the surface of the earth to the equator,

But

while that which is heated is carried along the higher strata to the poles, forming two counter currents in the direction of the meridian. the rotatory velocity of the air, corresponding to its geographical position, decreases towards the poles; in approaching the equator, it must therefore revolve more slowly than the corresponding parts of the earth, and the bodies on the surface of the earth must strike against it with the excess of their velocity, and, by its reaction, they will meet with a resistance contrary to their motion of rotation so that the wind will appear, to a person supposing himself to be at rest, to blow in a contrary direction to the earth's rotation, or from east to west, which is the direction of the trade winds.

The equator does not exactly coincide with the line which separates the trade winds north and south of it; that line of separation depends upon the total difference of heat in the two hemispheres, arising from the unequal length of their summers, the distribution of land and water, and other causes. There are many proofs of the existence of a counter current above the trade winds. On the Peak of Teneriffe, the prevailing winds are from the west. The ashes of the volcano of St. Vincent's, in the year 1812, were carried to windward as far as the island of Barbadoes by the upper The captain of a Bristol ship declared

current.

that, on that occasion, dust from St. Vincent's fell to the depth of five inches on the deck at the distance of 500 miles to the eastward; and light clouds have frequently been seen moving rapidly from west to east, at a very great height above the trade winds, which were sweeping along the surface of the ocean in a contrary direction.

SECTION XVII.

WITHOUT the atmosphere, death-like silence would prevail through nature, for it, in common with all substances, has a tendency to impart vibrations to those in contact with it, therefore undulations received by the air, whether it be from a sudden impulse, such as an explosion, or the vibrations of a musical chord, are propagated equally in every direction, and produce the sensation of sound upon the auditory nerves. In the small undulations of deep water in a calm, the vibrations of the liquid particles are made in the vertical plane, that is, at right angles to the direction of the transmission of the waves; but the vibrations of the particles of air which produce sound differ, being performed in the same direction in which the waves of sound travel. The propagation of sound may be illustrated by a field of corn agitated by a gust of wind; for however irregular the motion of the corn may seem, on a superficial view, it will be

to be no limit to the ascent of the liquid. The phenomena arising from the force of cohesion are innumerable: the spherical form of rain-drops and shot, the rise of liquids between plane surfaces, the difficulty of detaching a plate of glass from the surface of water, the force with which two plane surfaces adhere when pressed together,—are all effects of cohesion, entirely independent of atmospheric pressure, and are included in the same analytical formulæ, which express all the circumstances accurately, although the law according to which the forces of cohesion and repulsion vary is unknown, except that they only extend to insensible distances.

The difference between the forces of cohesion and repulsion is called molecular force, and, when modified by the electrical state of the particles, is the general cause of chemical affinities, which only take place between particles of different kinds of matter, though not under all circumstances. Two substances may indeed be mixed, but they will not combine to form a third substance different from both, unless their component particles unite in definite proportions. That is to say-one volume of one of the substances will unite with one volume of the other, or with two volumes, or with three, &c., so as to form a new substance, but in any other proportions it will only form a mixture of the two. For example, one volume of hydrogen gas will combine with eight volumes of oxygen, and form water; or