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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 320
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 2390 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 771⁄2 degrees below that point at the equator, and 119 below it at the poles, with a range of temperature from the surface of 1613° 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
sulted, the determination becomes more complicated. Mr. Ivory's method of computing heights from barometrical measurements 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. Admiral Smyth, R.N., and Sir John Herschel measured the height of Etna by the barometer, without any communication and in different years; Admiral Smyth made it 10,874 feet, and Sir John Herchel 10,873, the difference being only one foot. In consequence of the diminished pressure of the atmosphere water boils at a lower temperature on mountain tops than in the valleys, which induced Fahrenheit to propose this mode of observation as a method of ascertaining their heights. It is very simple, as Professor Forbes ascertained that the temperature of the boiling point varies in arithmetical proportion with the height, or 5495 feet for every degree of Fahrenheit, so that the calculation of height becomes one of arithmetic only, without the use of any table.
The mean pressure of the atmosphere is not the same all over the globe. It is less by 0.24 of an inch at the equator than at the tropics or in the higher latitudes, in consequence of the ascent of heated air and vapour from the surface of the ocean. It is less also on the shores of the Baltic Sea than it is in France, and it was observed by Sir James C. Ross that throughout the whole of the Antarctic Ocean, from 68° to 74° S. latitude, and from 8° to 7° W. longitude, there is a depression of the barometer amounting to an inch and upwards, which is equivalent to an elevation above the sea level of 800 feet. A similar depression was observed by M. Erman in the sea of Ochotzk, and in the adjacent continent of eastern Siberia. Sir John Herschel assigns as the cause of these singular anomalies the great system of circulation of the trade and antetrade winds, in both hemispheres, reacting upon the general mass of the continents as obstacles in their path, which is modified by the configuration of the land.
There are various periodic oscillations in the atmosphere, which, rising and falling like waves in the sea, occasion corresponding changes in the height of the barometer, but they differ as much from the trade-winds, monsoons, and other currents, as the tides of the sea do from the Gulf-stream and other oceanic rivers.
The sun and moon disturb the equilibrium of the atmosphere by their attraction, and produce annual undulations which have their maximum altitudes at the equinoxes, and their minima at the solstices. There are also lunar tides, which ebb and flow twice in the course of a lunation. The diurnal tides, which accomplish their rise and fall in six hours, are greatly modified by the heat of the sun. Between the tropics the barometer attains its maximum height about nine in the morning, then sinks till three or four in the afternoon; it again rises and attains a second maximum about nine in the evening, and then it begins to fall, and reaches a second minimum at three in the morning, again to pursue the same course. According to M. Bouvard, the amount of the oscillations at the equator is proportional to the temperature, and in other parallels it varies as the temperature and the square of the cosine of the latitude conjointly; consequently it decreases from the equator to the poles, but it is somewhat greater in the day than in the night.
Besides these small undulations, there are vast waves perpetually moving over the continents and oceans in separate and independent systems, being confined to local, yet very extensive districts, probably occasioned by long-continued rains or dry weather over large tracts of country. By numerous barometrical observations made simultaneously in both hemispheres, the courses of several have been traced, some of which occupy twenty-four, and others thirty-six, hours to accomplish their rise and fall. One especially of these vast barometric waves, many hundreds of miles in breadth, has been traced over the greater part of Europe; and not its breadth only, but also the direction of its front and its velocity, have been clearly ascertained. Although, like all other waves, these are but moving forms, yet winds arise dependent on them like tide streams in the ocean. Mr. Birt has determined the periods of other waves of still greater extent and duration, two of which required seventeen days to rise and fall; and another which takes fourteen days to complete its undulation, called by Mr. Birt the November wave, passes annually over the British Islands, probably over the whole of Europe and the seas on its northern coasts. Its crest, which appears to be 6000 miles in extent, moves from N.W. to S.E. at the rate of about 19 miles an hour; while the extent of its barometrical elevation from its trough to its crest is
seldom less than an inch, sometimes double that quantity. The great crest is preceded and followed at about five days' interval by two lower ones, and the beginning and end are marked by deep depressions. The researches of M. Leverrier leave no doubt that the great Crimean storm of the 14th November, 1854, was part of this phenomenon,* for even a very small difference of atmospheric pressure is sufficient to raise a considerable wind. Since each oscillation has its perfect effect independently of the others, each one is marked by a change in the barometer, and this is beautifully illustrated by curves constructed from a series of observations. The general form of the curve shows the course of the principal wave, while small undulations in its outline mark the maxima and minima of the minor oscillations.
The trade-winds, which are the principal currents in the atmosphere, are only a particular case of those very general laws which regulate the motion of the winds depending on the rarefaction of the air combined with the rotation of the earth on its axis. They are permanent currents of wind between the tropics, blowing to the N.E. on the N. side of the equator, and to the S.E. on the S. side.
If currents of air come from the poles, it is clear that equilibrium must be restored by counter-currents from the equator; moreover, winds coming from the poles, where there is no rotation, to the equator, which revolves from W. to E. at the rate of 1000 miles an hour, must of necessity move in a direction resulting from their own progressive motion and that of rotation; hence, in blowing towards the equator the bias is to the E., and in blowing from it the bias is to the W. Thus as N. and S. winds from the poles blow along the surface from the tropics to the equator, in consequence of this composition of motions that from the N. becomes the N.E. trade-wind, and that from the S. the S.E. trade-wind. Now these winds being in contrary directions cross at the equator, balance each other through about 6 degrees of latitude, and produce a belt of calms of that breadth encircling the globe, known as the calms of the equator, or the Variables of seamen. The heat of the sun rarefies the air so much, that the trade-winds, after crossing at the equator, ascend into the higher regions of the atmosphere, where that from the N. goes to the tropic of Capricorn, and * Sir John Herschel on Meteorology.