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of 11449000000 hundreds of millions of pounds. fish, which have the power of producing a vacuum, adhere to the rocks by a pressure of fifteen pounds upon every square inch of contact.

Since the atmosphere is both elastic and heavy, its density necessarily diminishes in ascending above the surface of the earth, for each stratum of air is compressed only by the weight above it; therefore the upper strata are less dense, because they are less compressed than those below them. Whence it is easy to show, supposing the temperature to be constant, that, if the heights above the earth be taken in increasing arithmetical progression,—that is, if they increase by equal quantities, as by a foot or a mile, the densities of the strata of air, or the heights of the barometer, which are proportional to them, will decrease in geometrical progression. For example, at the level of the sea, if the mean height of the barometer be 29-922 inches, at the height of 18000 feet it will be 14.961 inches, or onehalf as great; at the height of 36000 feet it will be onefourth as great; at 54000 feet it will be one-eighth, and so on, which affords a method of measuring the heights of mountains with considerable accuracy, and would be very simple if the decrease in the density of the air were exactly according to the preceding law, but it is modified by several circumstances, and chiefly by the changes of temperature, because heat dilates the air and cold contracts it, the variation being for every degree of Fahrenheit's thermometer. Experience shows that the heat of the air decreases as the height above the surface of the earth increases; and it appears, from recent investigations, that the mean temperature of space is 58° below the zero point of Fahrenheit, which would probably be the temperature

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of the surface of the earth also, were it not for the nonconducting power of the air, whence it is enabled to retain the heat of the sun's rays, which the earth imbibes and radiates in all directions. The decrease in heat is very irregular, but from the mean of many observations, it appears to be about 14° or 15° for every 9843 feet, which is the cause of the severe cold and eternal snows on the summits of the Alpine chains. The expansion of the atmosphere from the heat of the sun occasions diurnal variations in the height of the barometer. Of the various methods of computing heights from barometrical measurements, that of Ivory has the advantage of combining accuracy with the greatest simplicity. The most remarkable result of barometrical measurement was recently obtained by Baron Von Humboldt, showing that about eighteen thousand square leagues of the northwest of Asia, including the Caspian Sea and the Lake of Aral, are more than three hundred and twenty feet below the level of the surface of the ocean in a state of mean equilibrium. This enormous basin is similar to some of those large cavities on the surface of the moon, and is attributed, by Humboldt, to the upheaving of the surrounding moun tain chains of the Himalaya, of Kuen-Lun, of Thian-Chan, to those of Armenia, of Erzerum, and of Caucasus, which, by undermining the country to so great an extent, caused it to settle below the usual level of the sea. The very contemplation of the destruction that would ensue from the bursting of any of those barriers which now shut out the sea is fearful. In consequence of the diminished pressure of the atmosphere, water boils at a lower temperature on the mountain tops than in the valleys, which induced Fahrenheit to propose this mode of observation as a

method of ascertaining their heights; but although an instrument was constructed for that purpose by Archdeacon Wollaston, it does not appear to have been much employed.

The atmosphere, when in equilibrio, is an ellipsoid flattened at the poles from its rotation with the earth: in that state its strata are of uniform density at equal heights above the level of the sea, and it is sensibly of finite extent, whether 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 rarifying 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, while that which is heated is carried along the higher strata to the poles, forming two counter currents in the direction of the meridian. But the rotary 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 current. The captain of a Bristol ship declared 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