parates them. At such a distance they neither attract nor repel one another; but the instant they are brought so near as to make the level part of the liquid disappear, and the two curved parts of it meet, the two plates will rush towards each other and remain pressed together. If one of the surfaces be wet and the other dry, they will repel one another when so near as to have a curved surface of liquid between them; but if forced to approach a little nearer the repulsion will be overcome, and they will attract each other as if they were both wet or both dry. Two balls of pith or wood floating in water, or two balls of tin floating in mercury, attract one another as soon as they are so near that the surface of the liquid is curved between them. Two ships in the ocean may be brought into collision by this principle. But two balls, one of which is wet and the other dry, repel one another as soon as the liquid which separates them is curved at its surface. A bit of tea leaf is attracted by the edge of the cup if wet and repelled when dry, provided it be not too far from the edge and the cup moderately full; if too full, the contrary takes place. It is probable that the rise of the sap in vegetables is in some degree owing to capillarity.

1 Note 172.

SECTION XV.

ANALYSIS OF THE ATMOSPHERE. ITS PRESSURE. LAW OF DECREASE IN DENSITY. LAW OF DECREASE IN TEMPERATURE. MEASUREMENT OF HEIGHTS BY THE BAROMETER. EXTENT OF THE ATMOSPHERE. OSCILLATIONS. BAROMETRICAL VARIATIONS CORRESPONDING TO PHASES OF THE MOON NOT OWING TO GRAVITATION, TRADE WINDS. COUNTER CURRENTS.

THE atmosphere is not homogeneous. It appears from analysis that of 100 parts 79 are azotic gas, and 21 oxygen, the great source of combustion and animal heat. Besides these there are three or four parts of carbonic acid gas in 1000 parts of atmospheric air. These proportions are found to be the same at all heights hitherto attained by man. The air is an elastic fluid resisting pressure in every direction, and is subject to the law of gravitation. As the space in the top of the tube of a barometer is a vacuum, the column of mercury suspended by the pressure of the atmosphere on the surface of the cistern is a measure of its weight. Consequently every variation in the density occasions a corresponding rise or fall in the barometrical column. The pressure of the atmosphere is about fifteen pounds on every square inch; so that the surface of the whole globe sustains a weight of 11,449,000,000 hundreds of millions of pounds. Shell-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 progressionthat 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 proportionate 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 18,000 feet it will be 14.961 inches or one half as great; at the height of 36,000 feet, it will be one fourth as great; at 54,000 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 changes of temperature, because heat dilates the air and cold contracts it, varying of the whole bulk when at 32°, 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 of the surface of the earth also were it not for the non-conducting 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 ; each authority gives a different estimate; probably because the decrease varies with the latitude as well as the height, and something is due also to local circumstances. But from the mean of five different statements, it seems to be about one degree for every 334 feet, which is the cause

4

the Alpine chains. Of the various methods of computing heights from barometrical measurements, that of Mr. Ivory has the advantage of combining accuracy with the greatest simplicity. Indeed the accuracy with which the heights of mountains can be obtained by this method is very remarkable. Capt. Smyth, R. N. and Sir John Herschel measured the height of Etna by the barometer without any communication and in different years; Capt. Smyth made it 10,874 feet, and Sir John Herschel 10,873; the difference being only one foot. 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; at that height it must be of ex

treme tenuity, for the decrease in density is so rapid that three fourths of all the air contained in the atmosphere is within four miles of the earth; and, as its superficial extent is 200 millions of square miles, its relative thickness is less than that of a sheet of paper when compared with its breadth. The air even on 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 which Marco Polo the Venetian felt on the mountains of Central Asia. M. Gay-Lussac and M. Biot ascended in a balloon to the height of 4.36 miles, which is the greatest elevation that man has attained, and they suffered greatly from the rarity of the air. It is true that at the height of thirty-seven miles, the atmosphere is still dense enough to reflect the rays of the sun when 18° below the horizon; but the tails of comets show that extremely attenuated matter is capable of reflecting light. 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 expansion of the atmosphere from the heat of the sun occasions diurnal variations in the height of the barometer. There are nocturnal oscillations also as regular as those of the day, though not to the same extent.

The sun and moon disturb the equilibrium of the atmosphere by their attraction, producing oscillations similar