Liebig remarks (in his “Animal Chemistry”) that in an equal number of respirations,[[418]] we consume a larger amount of oxygen at the level of the sea than on a mountain; and it can be shown that under ordinary circumstances at Dorjiling, 20·14 per cent. less is inhaled than on the plains of India. Yet the chest cannot expand so as to inspire more at once, nor is the respiration appreciably quickened; by either of which means nature would be enabled to make up the deficiency. It is true that it is difficult to count one’s own respirations, but the average is considered in a healthy man to be eighteen in a minute; in my own case it is sixteen, an acceleration of which by three or four could not have been overlooked, in the repeated trials I made at Dorjiling, and still less the eight additional inhalations required at 15,000 feet to make up for the deficiency of oxygen in the air of that elevation.
[418] For the following note I am indebted to my friend, C. Muller, Esq., of Patna.—
According to Sir H. Davy, a man consumes 45,504 cubic inches of oxygen in twenty-four hours, necessitating the inspiration of 147,520 cubic inches of atmospheric air.—At pressure 23 inches, and temp. 60° this volume of atmospheric air (dry) would weigh 35,138·75 grains.—At pressure 30 in., temp. 80°, it would weigh 43,997·63 gr.
The amount of oxygen in atmospheric air is 23·32 per cent. by weight. The oxygen, then, in 147,520 cubic inches of dry air, at pressure 23 in., temp. 80°, weighs 8,194·35 gr.; and at pressure 30 in., temp. 80°, it weighs 10,260·25 gr.
Hence the absolute quantity of oxygen in a given volume of atmospheric air, when the pressure is 23 in., and the temp. 60°, is 20·14 per cent. less than when the pressure is 30 in. and the temp. 80°.
When the air at pressure 23 in:, temp. 60°, is saturated with moisture, the proportion of dry air and aqueous vapour in 100 cubic inches is as follows:—
Dry air 97·173
Vapour 2·827
At pressure 30 in., temp. 80°, the proportions are:—
Dry air 96·133
Vapour 3·867
The effect of aqueous vapour in the air on the amount of oxygen available for consumption, is very trifling; and it must not be forgotten that aqueous vapour supplies oxygen to the system as well as atmospheric air.
It has long been surmised that an alpine vegetation may owe some of its peculiarities to the diminished atmospheric pressure; and that the latter being a condition which the gardener cannot supply, he can never successfully cultivate such plants in general. I know of no foundation for this hypothesis; many plants, natives of the level of the sea in other parts of the world, and some even of the hot plains of Bengal, ascend to 12,000 and even 15,000 feet on the Himalaya, unaffected by the diminished pressure. Any number of species from low countries may be cultivated, and some have been for ages, at 10,000 to 14,000 feet without change. It is the same with the lower animals; innumerable instances may with ease be adduced of pressure alone inducing no appreciable change, whilst there is absence of proof to the contrary. The phenomena that accompany diminished pressure are the real obstacles to the cultivation of alpine plants, of which cold and the excessive climate are perhaps the most formidable. Plants that grow in localities marked by sudden extremes of heat and cold, are always very variable in stature, habit, and foliage. In a state of nature we say the plants “accommodate themselves” to these changes, and so they do within certain limits; but for one that survives of all the seeds that germinate in these inhospitable localities, thousands die. In our gardens we can neither imitate the conditions of an alpine climate, nor offer others suited to the plants of such climates.
The mean height of the barometer at Mr. Hodgson’s was 23·010, but varied 0·161 between July, when it was lowest, and October, when it was highest; following the monthly rise and fall of Calcutta as to period, but not as to amount (or amplitude); for the mercury at Calcutta stands in July upwards of half an inch (0·555 Prinsep) lower than it does in December.
The diurnal tide of atmosphere is as constant as to the time of its ebb and flow at Dorjiling as at Calcutta; and a number of very careful observations (made with special reference to this object) between the level of the plains of India, and 17,000 feet, would indicate that there is no very material deviation from this at any elevation in Sikkim. These times are very nearly 9.50 a.m. and about 10 p.m. for the maxima, the 9.50 a.m. very constantly, and the 10 p.m. with more uncertainty; and 4 a.m. and 4 p.m. for the minima, the afternoon ebb being most true to its time, except during the rains.
At 9.50 a.m. the barometer is at its highest, and falls till 4 p.m., when it stands on the average of the year 0·074 of an inch lower; during the same period the Calcutta fall is upwards of one-tenth of an inch (0·121 Prinsep).
It has been proved that at considerable elevations in Europe, the hours of periodic ebb and flow differ materially from those which prevail at the level of the sea; but this is certainly not the case in the Sikkim Himalaya.
The amplitude decreases in amount from 0·100 at the foot of the hills, to 0·074 at 7000 feet; and the mean of 132 selected unexceptionable observations, taken at nine stations between 8000 and 15,500 feet, at 9.50 a.m. and 4 p.m., gives an average fall of 0·056 of an inch; a result which is confirmed by interpolation from numerous horary observations at these and many other elevations, where I could observe at the critical hours.
That the Calcutta amplitude is not exceptionally great, is shewn by the register kept at different places in the Gangetic valley and plains of India, between Saharunpore and the Bay of Bengal. I have seen apparently trustworthy records of seven[[419]] such, and find that in all it amounts to between 0·084 and 0·120 inch, the mean of the whole being 0·101 of an inch.
[419] Calcutta, Berampore, Benares, Nagpore, Moozufferpore, Delhi, and Saharunpore.