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in the mercury of the barometer when rising. The absorption of moisture by sponges, sugar, salt, &c., are familiar examples of capillary attraction. Indeed the pores of sugar are so minute, that there seems to be no limit to the ascent of the liquid. Wine is drawn up in a curve on the interior surface of a glass; tea rises above its level on the side of a cup; but, if the glass or cup be too full, the edges attract the liquid downwards, and give it a rounded form. A column of liquid will rise above or sink below its level between two plane parallel surfaces when near to one another, according to the relative densities of the plates and the liquid (N. 175); and the phenomena will be exactly the same as in a cylindrical tube whose diameter is double the distance of the plates from each other. If the two surfaces be very near to one another, and touch each other at one of their upright edges, the liquid will rise highest at the edges that are in contact, and will gradually diminish in height as the surfaces become more separated. The whole outline of the liquid column will have the form of a hyperbola. Indeed, so universal is the action of capillarity, that solids and liquids cannot touch one another without producing a change in the form of the surface of the liquid.
The attractions and repulsions arrising from capillarity present many curious phenomena. If two plates of glass or metal, both of which are either dry or wet, be partly immersed in a liquid parallel to one another, the liquid will be raised or depressed close to their surfaces, but will maintain its level through the rest of the space that separates 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 (N. 176). 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
But two balls, one of
brought into collision by this principle. 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.
Analysis of the Atmosphere
Its Pressure Law of Decrease in Density
Law of Decrease in Temperature Barometer Extent of the Atmosphere Oscillations Trade-Winds Cloud-Ring Monsoons Rotation of Winds Laws of Hurricanes.
THE atmosphere is not homogeneous. It appears from analysis that, of 100 parts, 99.5 consist of nitrogen and oxygen gases mixed in the proportions of 79 to 21 of volume, the remainder consists of 0.05 parts of carbonic acid and on an average 0.45 of aqueous vapour. 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 that in the cistern is a measure of its weight. Consequently every variation in the density occasions a corresponding rise or fall in the barometrical column. At the level of the sea in latitude 420, and at the temperature of melting ice, the mean height of the barometer is 29.922 or 30 inches nearly. 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 11,671,000,000 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.
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; but since the air is both heavy and elastic, 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 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. Sir John Herschel has shown that the actual decrease is much more rapid, and that, in any hypothesis that has been formed with regard to the divisibility of the aërial atoms, a vacuum exists at the height of 80 or 90 miles above the earth's surface, inconceivably more perfect than any that can be produced in the best air-pumps. Indeed 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 ascended in a balloon to the height of 4.36 miles, and he 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 density of the air is modified by various circumstances, chiefly by changes of temperature, because heat dilates the air and cold contracts it, varying of the whole bulk when at 329
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. It appears that the mean temperature of space is 2260 below the zero point of Fahrenheit by the theories of Fourier and Pouillet, but Sir John Herschel has computed it to be 239° Fahr. from observations made during the ascent in balloons. Such 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, because it varies with latitude and local circumstances, but from the mean of five different statements it seems to be about one degree for every 334 feet; the mean of observations made in balloons is 400 feet, which is probably nearer the truth. This is the cause of the severe cold and perpetual snow on the summits of the alpine chains. In the year 1852 four ascents in a balloon took place from the meteorological observatory at Kew, in which the greatest height attained was 22,370 feet. The observations then made by Mr. Welsh furnished Sir John Herschel with data for computing that the temperature of space is minus 2390, that is 239° below the zero point of Fahrenheit, that the limiting temperature of the atmosphere is probably 77 degrees below that point at the equator, and 119 below it at the poles, with a range of temperature from the surface of 16130 in the former case, and 11940 in the latter. During these ascents it was found that the temperature of the air decreases uniformly up to a certain point, where it is arrested and remains constant, or increases through a depth of 2000 or 3000 feet, after which it decreases again according to the same law as before. Throughout this zone of constant temperature it either rains, or there is a great fall in the dew point; in short, it is the region of clouds, and the increase of temperature is owing to the latent or absorbed heat set free by the condensation of the aqueous vapour. In the latitude of Kew the cloud region begins at altitudes varying between 2000 and 6500 feet, according to the state of the weather.
Were it not for the effects of temperature on the density of the air, the heights of mountains might be determined by the barometer alone; but as the thermometer must also be con