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214 THE ATMOSPHERE AND METEOROLOGY.
many mountains raise their peaks, the column of air which rests on the
ground has already lost one-half of its weight; consequently, all the gaseous mass which extends far into the sky to. immeasurable distances, is
simply equal to the aerial strata compressed into this lower region.
More than two hundred years ago, Perier, following the indications of
his brother-in-law Pascal, established by the first direct experiment the
diminution of the weight of the air in a vertical direction: he ascended
Puy-de-D6me with the barometer in his hand, and during the ascent the
column of mercury which measured the atmospheric pressure never ceased
to sink gradually in the tube, and thus the means of measuring the height
of mountains above the level of the sea, by simply reading the barometrical indications, was discovered. Since this epoch science has made great
progress, the precise law of the decrease of the weight of the air and all
other elastic gases has been brought to light by Mariotte, and innumerable travelers have been able, with the aid of the barometer, to indicate
approximately the altitude of the salient points in the various countries
that they have traversed. Nevertheless, one can never be sure that the
barometer has furnished perfectly exact measures of height. In each
barometric reading we must take into account the temperature, the quantity of watery vapor contained in the atmosphere, the agitation of the
winds—in a word, all those physical conditions of the air whose weight
we are about to measure, and each of these secondary observations makes
a greater or less correction necessary in the final result. The direct results obtained by trigonometry are at present the only ones that give in
an exact manner the height of the surface.
To ascertain the altitude of summits another means is also employed,
which, in consequence of the defectiveness of the instruments, generally
gives results still less exact than those of the barometer. This means
consists in measuring the heat of boiling water. In fact, the boiling-
point, or the temperature at which the tension of the vapor of water exactly balances the atmospheric pressure, must necessarily sink in proportion as the pressure diminishes. It has been calculated that the average fall of boiling-point is 18° Fahrenheit for every 1062 feet of vertical
height. But experiments may give for the heights of mountains differences of many hundred feet. Thus, Tyndall found, in August, 1859, that
the temperature of boiling water on the summit of Mont Blanc was 84*97°,
while in the preceding year he had observed a slightly lower boiling-
point on Mont Rosa, though this latter peak is five hundred and fifty-eight
feet lower than the giant of the Alps.
To what height is the air dense enough for a man to be able to find the
oxygen necessary for his lungs, and to live there for a few seconds at
least ? The climbers of mountains have never reached this extreme limit,
because of the fatigues of the ascent, which add to their difficulty of finding a sufficient quantity of air. Thus the highest p*eaks of the Himalayas and the Andes have remained to this day untrodden by human foot.*
* See The Earth, the section entitled Mountains.
| Title | The ocean, atmosphere, and life |
| Creator | Reclus, Elisée |
| Publisher | Harper |
| Place of Publication | New York |
| Date | 1873 |
| Language | eng |
| Type | Books/Pamphlets |
| Title | 00000237 |
| Type | Books/Pamphlets |
| Transcript | 214 THE ATMOSPHERE AND METEOROLOGY. many mountains raise their peaks, the column of air which rests on the ground has already lost one-half of its weight; consequently, all the gaseous mass which extends far into the sky to. immeasurable distances, is simply equal to the aerial strata compressed into this lower region. More than two hundred years ago, Perier, following the indications of his brother-in-law Pascal, established by the first direct experiment the diminution of the weight of the air in a vertical direction: he ascended Puy-de-D6me with the barometer in his hand, and during the ascent the column of mercury which measured the atmospheric pressure never ceased to sink gradually in the tube, and thus the means of measuring the height of mountains above the level of the sea, by simply reading the barometrical indications, was discovered. Since this epoch science has made great progress, the precise law of the decrease of the weight of the air and all other elastic gases has been brought to light by Mariotte, and innumerable travelers have been able, with the aid of the barometer, to indicate approximately the altitude of the salient points in the various countries that they have traversed. Nevertheless, one can never be sure that the barometer has furnished perfectly exact measures of height. In each barometric reading we must take into account the temperature, the quantity of watery vapor contained in the atmosphere, the agitation of the winds—in a word, all those physical conditions of the air whose weight we are about to measure, and each of these secondary observations makes a greater or less correction necessary in the final result. The direct results obtained by trigonometry are at present the only ones that give in an exact manner the height of the surface. To ascertain the altitude of summits another means is also employed, which, in consequence of the defectiveness of the instruments, generally gives results still less exact than those of the barometer. This means consists in measuring the heat of boiling water. In fact, the boiling- point, or the temperature at which the tension of the vapor of water exactly balances the atmospheric pressure, must necessarily sink in proportion as the pressure diminishes. It has been calculated that the average fall of boiling-point is 18° Fahrenheit for every 1062 feet of vertical height. But experiments may give for the heights of mountains differences of many hundred feet. Thus, Tyndall found, in August, 1859, that the temperature of boiling water on the summit of Mont Blanc was 84*97°, while in the preceding year he had observed a slightly lower boiling- point on Mont Rosa, though this latter peak is five hundred and fifty-eight feet lower than the giant of the Alps. To what height is the air dense enough for a man to be able to find the oxygen necessary for his lungs, and to live there for a few seconds at least ? The climbers of mountains have never reached this extreme limit, because of the fatigues of the ascent, which add to their difficulty of finding a sufficient quantity of air. Thus the highest p*eaks of the Himalayas and the Andes have remained to this day untrodden by human foot.* * See The Earth, the section entitled Mountains. |
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