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- Himalayan Journals (Complete) - 130/157 -

During the rains, 3 degrees to 4 degrees is the mean maximum, but the nights being almost invariably cloudy, it is scarcely on one night out of six that there is any radiation. From October to December the amount is greater = 10 degrees to 12, and from January till May greater still, being as much as 15 degrees. During the winter months the effect of radiation is often felt throughout the clear days, dew forming abundantly at 4000 to 8000 feet in the shaded bottoms of narrow valleys, into which the sun does not penetrate till 10 a.m., and from which it disappears at 3 p.m. I have seen the thermometer in the reflector fall 12 degrees at 10 a.m. in a shaded valley. This often produces an anomalous effect, causing the temperature in the shade to fall after sunrise; for the mists which condense in the bottom of the valleys after midnight disperse after sunrise, but long before reached by the sun, and powerful radiation ensues, lowering the surrounding temperature: a fall of 1 degree to 2 degrees after sunrise of air in the shade is hence common in valleys in November and December.* [Such is the explanation which I have offered of this phenomenon in the Hort. Soc. Journal. On thinking over the matter since, I have speculated upon the probability of this fall of temperature being due to the absorption of heat that must become latent on the dispersion of the dense masses of white fog that choke the valleys at sunrise.] The excessive radiation of the winter months often gives rise to a curious phenomenon; it causes the formation of copious dew on the blanket of the traveller's bed, which radiates heat to the tent roof, and this inside either an open or a closed tent. I have experienced this at various elevations, from 6000 to 16,000 feet. Whether the minimum temperature be as high as 50 degrees, or but little above zero, the effect is the same, except that hoar-frost or ice forms in the latter case. Another remarkable effect of nocturnal radiation is the curl of the alpine rhododendron leaves in November, which is probably due to the freezing and consequent expansion of the water in the upper strata of cells exposed to the sky. The first curl is generally repaired by the ensuing day's sun, but after two or three nights the leaves become permanently curled, and remain so till they fall in the following spring.

I have said that the nocturnal radiation in the English spring months is the great obstacle to the cultivation of many Himalayan plants; but it is not therefore to be inferred that there is no similar amount of radiation in the Himalaya; for, on the contrary, in April its amount is much greater than in England, frequently equalling 13 degrees of difference; and I have seen 16 degrees at 7,500 feet; but the minimum temperature at the time is 51 degrees, and the absolute amount of cold therefore immaterial. The mean minimum of London is 38 degrees, and, when lowered 5.5 degrees by radiation, the consequent cold is very considerable. Mr. Daniell, in his admirable essay on the climate of London, mentions 17 degrees as the maximum effect of nocturnal radiation ever observed by him. I have registered 16 degrees in April at Dorjiling; nearly as much at 6000 feet in February; twice 13 degrees, and once 14.2 degrees in September at 15,500 feet; and 10 degrees in October at 16,800 feet; nearly 13 degrees in January at 7000 feet; 14.5 degrees in February at that elevation, and, on several occasions, 14.7 degrees at 10,000 feet in November.

The annual rain-fall at Dorjiling averages 120 inches (or 10 feet), but varies from 100 to 130 in different years; this is fully three times the amount of the average English fall,* [The general ideas on the subject of the English rain-fall are so very vague, that I may be pardoned for reminding my readers that in 1852, the year of extraordinary rain, the amounts varied from 28.5 inches in Essex, to 50 inches at Cirencester, and 67.5 (average of five years) at Plympton St. Mary's, and 102.5 at Holme, on the Dart.] and yet not one-fourth of what is experienced on the Khasia hills in Eastern Bengal, where fifty feet of rain falls. The greater proportion descends between June and September, as much as thirty inches sometimes falling in one month. From November to February inclusive, the months are comparatively dry; March and October are characterised by violent storms at the equinoxes, with thunder, destructive lightning, and hail.

The rain-gauge takes no account of the enormous deposition from mists and fogs: these keep the atmosphere in a state of moisture, the amount of which I have estimated at 0.88 as the saturation-point at Dorjiling, 0.83 being that of London. In July, the dampest month, the saturation-point is 0.97; and in December, owing to the dryness of the air on the neighbouring plains of India, whence dry blasts pass over Sikkim, the mean saturation-point of the month sometimes falls as low as 0.69.

The dew-point is on the average of the year 49.3 degrees, or 3 degrees below the mean temperature of the air. In the dampest month (July) the mean dew-point is only eight-tenths of a degree below the temperature, whilst in December it sinks 10 degrees below it. In London the dew-point is on the average 5.6 degrees below the temperature; none of the English months are so wet as those of Sikkim, but none are so dry as the Sikkim December sometimes is.

_On the weight of the atmosphere in Sikkim; and its effects on the human frame._

Of all the phenomena of climate, the weight of the atmosphere is the most remarkable for its elusion of direct observation, when unaided by instruments. At the level of the sea, a man of ordinary bulk and stature is pressed upon by a superincumbent weight of 30,000 pounds or 13.5 tons. An inch fall or rise in the barometer shows that this load is lightened or increased, sometimes in a few hours, by nearly 1,000 pounds; and no notice is taken of it, except by the meteorologist, or by the speculative physician, seeking the subtle causes of epidemic and endemic complaints. At Dorjiling (7,400 feet), this load is reduced to less than 2,500 pounds, with no appreciable result whatever on the frame, however suddenly it be transported to that elevation. And the observation of my own habits convinced me that I took the same amount of meat, drink, sleep, exercise and work, not only without inconvenience, but without the slightest perception of my altered circumstances. On ascending to 14,000 feet, owing to the diminished supply of oxygen, exercise brings on vertigo and headache; ascending higher still, lassitude and tension across the forehead ensue, with retching, and a sense of weight dragging down the stomach, probably due to dilatation of the air contained in that organ. Such are the all but invariable effects of high elevations; varying with most persons according to the suddenness and steepness of the ascent, the amount and duration of exertion, and the length of time previously passed at great heights. After having lived for some weeks at 15,300 feet, I have thence ascended several times to 18,500, and once above 19,000 feet, without any sensations but lassitude and quickness of pulse;* [I have in a note to vol. ii. chapter xxiii, stated that I never experienced in my own person, nor saw in others, bleeding at the ears, nose, lips, or eyelids.] but in these instances it required great caution to avoid painful symptoms. Residing at 15,300 feet, however, my functions were wholly undisturbed; nor could I detect any quickness of pulse or of respiration when the body was at rest, below 17,000 feet. At that elevation, after resting a party of eight men for an hour, the average of their and my pulses was above 100 degrees, both before and after eating; in one case it was 120 degrees, in none below 80 degrees.

Not only is the frame of a transient visitor unaffected (when at rest) by the pressure being reduced from 30,000 to 13,000 pounds, but the Tibetan, born and constantly residing at upwards of 14,000 feet, differs in no respect that can be attributed to diminished pressure, from the native of the level of the sea. The averaged duration of life, and the amount of food and exercise is the same; eighty years are rarely reached by either. The Tibetan too, however inured to cold and great elevations, still suffers when he crosses passes 18,000 or 19,000 feet high, and apparently neither more nor less than I did.

Liebig remarks (in his "Animal Chemistry") that in an equal number of respirations,* [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 degrees this volume of atmospheric air (dry) would weigh 35,13875 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 degrees, weighs 8,194.35 gr.; and at pressure 30 in., temp. 80 degrees, 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 degrees, is 20.14 per cent. less than when the pressure is 30 in. and the temp. 80 degrees.

When the air at pressure 23 in:, temp. 60 degrees, 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 degrees, 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.] 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.

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

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