CHAPTER II.

19. Figure of the Atmosphere. 20. Its specific gravity. 21. Density decreases with ascent; physical effects therefrom. 22. Height computed by the Thermometer. 23. Boiling point of water under various atmospheric pressures. 24. Sufferings from rarefied air,-Gerard, Wood. 25. Batten, Moorcroft, and Hearsay; Liebig's explanation. 26. Effects of Altitude upon Sound. 27. Height of the atmosphere. 28. Computed by Refraction. 29. Mariotte's law. 30. Illustrated hypothetically. 31. Mean pressure; weight of the entire mass of air. 32. Pascal's experiment on the Puy de Dome; elasticity of the atmosphere. 33. Dove on barometric oscillations. 34. Diurnal tidal waves in the Barometer. 35. Anomalous observations: theory. 36. Other oscillations. 37. Diurnal variation. 38. Annual range. 39. Peculiarities at the Cape and Calcutta. 40. Isobarometric lines of Kamtz. 41. Anomalous movements of the Barometer. 42. Influence of Winds. 43. Explanation of barometric fluctuations.

19. The atmosphere which surrounds our globe is of a figure similar to the earth; hence spheroidal, the equatorial diameter being greater than the polar axis. Though these terrestrial diameters bear the proportion of 300 to 299, we cannot estimate the dimensions of the atmospheric sphere with certainty; the probability however is, that the equatorial bears a greater proportion to the polar diameter than the ratio given.

20. We find that the property of weight possessed by atmospheric air in common with other bodies, was known to the ancients, though subsequently abandoned. Aristotle' asserted that a bladder inflated with atmospheric air, weighed more than when empty. The weight of a given quantity has been differently computed by various observers. Perhaps it is not far from the truth, to say that 100 cubic inches at mean tem

perature and density, weigh from 30.2662 to 31.0 grains,' which is equal to 523 grains or 1.195 oz. avoid. for every cubic foot Prout calculates the weight of 100 cubic inches of dry air, the barometer being 30 inches and thermometer 60° F. at 31.0117 grains. Dalton and Henry compute it at 31; and Shuckburgh at 30.5 grains. Atmospheric air bears to water the relative weight of 0.00119 to 1.0, i.e. bulk for bulk, air is 840 times lighter than pure water. Hooke gives the proportion somewhat lower. On the 10th February 1664, he found that a pint of water weighed 8.21872 oz., and the same measure of air 8.28947 grains, and determined the specific gravity of air to be very nearly 0.0011765. According to Lavoisier, with the barometer at 29.85 inches and thermometer at 54.5°, the specific gravity of the atmosphere is 0.0012308, (vide 120).

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21. As we ascend, the atmosphere gradually decreases in density, and on lofty situations the effects of its rarity are disagreeably manifested. Acosta in the 16th century described the violent symptoms which he and his companions experienced on the mountains of Peru;' and very recently, Dr J. J. Von Tschudi," on the Cordilleras. Baron Humboldt, at an altitude of about 16,000 feet, felt overcome with fatigue, blood burst from his lips and ears, and respiration was affected. A fire was kindled and kept burning with greater difficulty than on the plain below, a fact which Marco Paulo observed on the mountains of Asia so early as the 13th century. Water, which in the plain boiled at the ordinary temperature of 212°

1 As 100 cubic inches of oxygen weigh 33.90 grains and 100 cubic inches of nitrogen weigh 29.655 grains, it follows that 100 cubic inches of atmospheric air {33.90+ (29.65 × 4)} ÷ 5 = 30.5 grains.

=

2 Phil. Tr. vol. lxvii. p. 560; Turner's Chem.

3 The sp. gr. of a substance equals its weight divided by its volume, hence, if x, w, v, and x', w', v', represent the sp. gr., weight and volume of air and water respec

tively, it follows that, bulk for bulk, æ: x'

1

1

weights are the same, x: x' =

:

ย

Ilence, the volumes being equal, the sp. gr.

is directly as the weight, and the weights being equal, inversely as the volume.

Hist. Nat. de las Indias.

Travels in Peru-Ross. Lond. 1847.

• Ed. Phil. Jour.; Athen. No. 524, p. 833.

B

F., boiled on one of the summits of the Andes at 175° F., where only 13ths of the atmosphere exerts its pressure. Saussure found water boil at 86.24° C., or nearly 187° F. on Mont Blanc; while in the valley below, it boiled at 101.62° C. or 214° 9 F., a difference of 27.684 degrees of Fahrenheit's thermometer. The barometer stood at 30.534 English inches on the plain, and 17.136 inches on the mountain top. On the Great St Bernard, elevated 11,006 feet, water boils at 190° F. On the summit of Ben-Nevis, elevated 4358 feet, it boils at 203.8° F., the temperature being 30° on the mountain, and 35° on the plain. A curious anecdote, shewing the effect of lofty elevation upon the temperature of boiling water, is told by Mr Darwin of the Beagle, who in 1835 crossed the Andes. The altitude was so great, and the boiling point so low, that "our potatoes, after remaining for some hours in the boiling water, were nearly as hard as ever. The pot was left on the fire all night, and next morning it was boiled again; but yet the potatoes were not cooked. I found out this, by overhearing my two companions discussing the cause; they had come to the simple conclusion, that the potatoes were bewitched, or that the pot, which was a new one, did not choose to boil them." On the Nonewara mountain, Baron Hügel had much difficulty in melting ice in consequence of its passing off in vapour without dissolving: at last he found the boiling point of water to be 188°.

22. In estimating the height of any mountain by the thermometer, a difference of one degree in the boiling point of water, between that upon the summit and in the plain, corresponds to an altitude of about 530 feet, the state of the atmosphere being unchanged during the observations. The Rev. F. J. H. Wollaston, by the invention of a thermometer, each degree of which is divided into 1000 parts, has brought this method to considerable perfection.

23. Seeing that the density of the atmosphere deter

According to Robison, fluids boil in vacuo at 140 degrees lower than in open air. Black's Lect. v. i. p. 151.

2 Trav. in Kashmér and the Panjab-Jervis, Lond. 1845, p. 167.

3 Phil. Tr. 1817.

mines the point of ebullition, the height of the barometer on the plain will indicate even there a variation in the boiling point. Thus, the mercury being at 27.7 English inches, water will boil at 208° F.; at 28.2 inches at 209°; at 28.8 inches at 210°; at 29.4 inches at 211°; at 30.0 inches at 212°; at 30.6 inches at 213°; at 31.2 inches at 214°; and at 31.8 inches at 215°.

24. The brothers Gerard, in their travels among the Himmalehs, frequently felt the inconvenience of atmospheric rarity. One of them thus describes his feelings:-"Our elevation was now upwards of 15,000 feet, although we had ascended in company with the river against its current. Here only began our toils, and we scaled the slope of the mountain slowly; respiration was laborious, and we felt exhausted at every step. The crest of the pass was not visible, and we saw no limit to our exertions. The road inclined at an angle of 30°, and passed under vast ledges of limestone. The projections frowned above us in new and horrid forms, and our situation was different from any thing we had yet experienced. Long before we got up, we were troubled with severe headaches, and our respiration became so hurried and oppressive that we were compelled to sit down every few yards, and even then we could scarcely inhale a sufficient supply of air. The least motion was accompanied with extreme debility, and a depression of spirits; and thus we laboured for two miles." Lieutenant Wood' at Pamer in central Asia,-the Bam-i-duniah, or roof of the world, (N. lat. 37° 27′, E. long. 73° 40′), perhaps the most lofty plateau on the globe,-endeavouring to break the ice on the lake of Sirikol, to measure its depth, found a few strokes with the axe exhausted the men, and continued work was impracticable. Mr Green and Mr Rush, who ascended in a balloon in Sept. 1838 to the height of 27,136 feet, or 5 miles above the sea, the greatest altitude we believe yet attained, -felt comparatively little inconvenience, though the first 11,000 feet were ascended in 7 minutes. This arose evidently from the almost absolute repose of the body. Mr Rush suffered

1 Asiatic Journal.

2 Personal Narrative of Journey to Source of the Oxus. Lond. 1841.

only from the cold, and Mr Green felt his respiration hurried only when he exerted himself.1

25. Captain Batten says the feelings experienced by him on the Nittee Pass were far more severe than angina pectoris. Moorcroft, describing an exploring expedition among the Himmalehs, when accompanied by Captain Hearsay and a Hindu, says that his breathing was quickened, and he was obliged to stop every four or five steps; he felt a sense of fulness in the head and giddiness. On ascending farther, the oppression increased, there was a sense of sleep, accompanied by a feeling of suffocation, and sighing became frequent and distressing. The natives attribute these sensations, which are felt by the animal creation likewise, to a poisonous atmosphere which they call Biskeehuiva, conceived to arise from the odours of certain flowers. Mr Lyell tells us that the English greyhounds, taken out for the Real del Monte Company in Mexico, when hunting at an altitude of 9000 feet, where the barometer does not rise above 19 inches, were unable to bear the fatigues of the chase, and fell down gasping in such an attenuated atmosphere; but, as if nature would provide for the altered condition of the race, the whelps felt no inconvenience from its rarity. The inhabitants of the Alps are said to be very subject to bronchial hæmoptysis from diminished atmospheric pressure. Although the late lamented Hon. James Erskine Murray did not perceive, on the Mont Perdu, at an altitude of 11,000 feet, any disagreeable inconvenience, yet, the elevations of European mountains and those of South America and Asia being so dissimilar, we must to that circumstance attribute in a great measure his freedom from the peculiar sensations which others have experienced. Dr Martin Barry has given the following account of his own feelings on ascending Mont Blanc in 1834. an altitude of 12,000 feet, and between that and 14,700, he

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1 Jameson's Jour. July 1840, vol. xxix. p. 44, note.

"Asiatic Researches, vol. xii.

"Hasse's Patholog. Anat. 1846, p. 242 ; Michéa-Gaz. des Hôpit., 1840, No. 41. Summer in Pyrenees, 2d ed. vol. ii. p. 68.

Ascent of Mont Blanc, 1836, p. 104.