Referring again to the cut, it must be borne in mind that neither of the two meridional ridges runs in a straight line, but that they wind or zigzag as all mountain ranges do; that spurs from each ridge are given off from either flank alternately, and that the origin of a spur on one side answers to the source of a river (_i.e.,_ the head of a valley) on the other. These rivers are feeders of the main stream, the Teesta, and run at more or less of an angle to the latter. The spurs from the east flank of one ridge cross, at their ends, those from the west flank of another; and thus transverse valleys are formed, presenting many modifications of climate with regard to exposure, temperature, and humidity.
The roads from the plains of India to the watershed in Tibet always cross these lateral spurs. The main ridge is too winding and rugged, and too lofty for habitation throughout the greater part of its length, while the river-channel is always very winding, unhealthy for the greater part of the year below 4000 feet, and often narrow, gorge-like, and rocky. The villages are always placed above the unhealthy regions, on the lateral spurs, which the traveller repeatedly crosses throughout every day"s march; for these spurs give off lesser ones, and these again others of a third degree, whence the country is cut up into as many spurs, ridges, and ranges, as there are rills, streams, and rivers amongst the mountains.
Though the direction of the main atmospheric current is to the north, it is in reality seldom felt to be so, except the observer be on the very exposed mountain tops, or watch the motions of the upper strata of atmosphere. Lower currents of air rush up both the main and lateral valleys, throughout the day; and from the sinuosities in the beds of the rivers, and the generally transverse directions of their feeders, the current often becomes an east or west one. In the branch valleys draining to the north the wind still ascends; it is, in short, an ascending warm, moist current, whatever course be pursued by the valleys it follows.
The sides of each valley are hence equally supplied with moisture, though local circ.u.mstances render the soil on one or the other flank more or less humid and favourable to a luxuriant vegetation: such differences are a drier soil on the north side, with a too free exposure to the sun at low elevations, where its rays, however transient, rapidly dry the ground, and where the rains, though very heavy, are of shorter duration, and where, owing to the capacity of the heated air for retaining moisture, day fogs are comparatively rare. In the northern parts of Sikkim, again, some of the lateral valleys are so placed that the moist wind strikes the side facing the south, and keeps it very humid, whilst the returning cold current from the neighbouring Tibetan mountains impinges against the side facing the north, which is hence more bare of vegetation. An infinite number of local peculiarities will suggest themselves to any one conversant with physical geography, as causing unequal local distribution of light, heat, and moisture in the different valleys of so irregular a country; namely, the amount of slope, and its power of retaining moisture and soil; the composition and hardness of the rocks; their dip and strike; the protection of some valleys by lofty snowed ridges; and the free southern exposures of others at great elevations.
The position and elevation of the perpetual snow* [It appears to me, as I have a.s.serted in the pages of my Journal, that the limit of perpetual snow is laid down too low in all mountain regions, and that acc.u.mulations in hollows, and the descent of glacial ice, mask the phenomenon more effectually than is generally allowed. In this work I define the limit, as is customary, in general terms only, as being that where the acc.u.mulations are very great, and whence they are continuous upwards, on gentle slopes. All perpetual snow, however, becomes ice, and, as such, obeys the laws of glacial motion, moving as a viscous fluid; whence it follows that the lower edge of a snow-bed placed on a slope is, in one sense, the termination of a glacier, and indicates a position below that where all the snow that falls melts. I am well aware that it is impossible to define the limit required with any approach to accuracy. Steep and broken surfaces, with favourable exposures to the sun or moist winds, are bare much above places where snow lies throughout the year; but the occurrence of a gentle slope, free of snow, and covered with plants, cannot but indicate a point below that of perpetual snow. Such is the case with the "Jardin" on the Mer de Glace, whose elevation is 9,500 feet, whereas that of perpetual snow is considered by Professor J.
Forbes, our best authority, to be 8,500 feet. Though limited in area, girdled by glaciers, presenting a very gentle slope to the east, and screened by surrounding mountains from a considerable proportion of the sun"s rays, the Jardin is clear, for fully three months of the year, of all but sporadic falls of snow, that never lie long; and so are similar spots placed higher on the neighbouring slopes; which facts are quite at variance with the supposition that the perpetual snow-line is below that point in the Mont Blanc Alps. On the Monte Rosa Alps, again, Dr. Thomson and I gathered plants in flower, above 12,000 feet on the steep face of the Weiss-thor Pa.s.s, and at 10,938 feet on the top of St. Theodule; but in the former case the rocks are too steep for any snow to lie, they are exposed to the south-east, and overhang a gorge 8000 feet deep, up which no doubt warm currents ascend; while at St. Theodule the plants were growing on a slope which, though gentle, is black and stony, and exposed to warm ascending currents, as on the Weiss-thor; and I do not consider either of these as evidences of the limit of perpetual snow being higher than their position.] vary with those of the individual ranges, and their exposure to the south wind. The expression that the perpetual snow lies lower and deeper on the southern slopes of the Himalayan mountains than on the northern, conveys a false impression.
It is better to say that the snow lies deeper and lower on the southern faces of the individual mountains and spurs that form the snowy Himalaya. The axis itself of the chain is generally far north of the position of the spurs that catch all the snow, and has comparatively very little snow on it, most of what there is lying upon north exposures.
A reference to the woodcut will show that the same circ.u.mstances which affect the distribution of moisture and vegetation, determine the position, amount, and duration of the snow. The princ.i.p.al fall will occur, as before shown, where the meridional range first attains a sufficiently great elevation, and the air becomes consequently cooled below 32 degrees; this is at a little above 14,000 feet, sporadic falls occurring even in summer at that elevation: these, however, melt immediately, and the copious winter falls also are dissipated before June. As the depth of rain-fall diminishes in advancing north to the higher parts of the meridional ranges, so does that of the snow-fall. The permanence of the snow, again, depends on--1. The depth of the acc.u.mulation; 2. The mean temperature of the spot; 3. The melting power of the sun"s rays; 4. The prevalence and strength of evaporating winds. Now at 14,000 feet, though the acc.u.mulation is immense, the amount melted by the sun"s rays is trifling, and there are no evaporating winds; but the mean temperature is so high, and the corroding powers of the rain (which falls abundantly throughout summer) and of the warm and humid ascending currents are so great, that the snow is not perennial.
At 15,500 feet, again, it becomes perennial, and its permanence at this low elevation (at P) is much favoured by the acc.u.mulation and detention of fogs over the rank vegetation which prevails from S nearly to P; and by the lofty mountains beyond it, which shield it from the returning dry currents from the north. In proceeding north all the circ.u.mstances that tend to the dispersion of the snow increase, whilst the fall diminishes. At P the deposition is enormous and the snow-line low--16,000 feet; whilst at T little falls, and the limit of perpetual snow is 19,000 and 20,000 feet. Hence the anomaly, that the snow-line ascends in advancing north to the coldest Himalayan regions. The position of the greatest peaks and of the greatest ma.s.s of perpetual snow being generally a.s.sumed as indicating a ridge and watershed, travellers, arguing from single mountains alone, on the meridional ridges, have at one time supported and at another denied the a.s.sertion, that the snow lies longer and deeper on the north than on the south slope of the Himalayan ridge.
The great acc.u.mulation of snow at 15,000 feet, in the parallel of P, exercises a decided influence on the vegetation. The alpine rhododendrons hardly reach 14,000 feet in the broad valleys and round-headed spurs of the mountains of the Tunkra and Chola pa.s.ses; whilst the same species ascend to 16,000, and one to 17,000 feet, at T. Beyond the latter point, again, the great aridity of the climate prevents their growth, and in Tibet there are generally none even as low as 12,000 and 14,000 feet. Glaciers, again, descend to 15,000 feet in the tortuous gorges which immediately debouch from the snows of Kinchinjunga, but no plants grow on the debris they carry down, nor is there any sward of gra.s.s or herbage at their base, the atmosphere immediately around being chilled by enormous acc.u.mulations of snow, and the summer sun rarely warming the soil. At T, again, the glaciers do not descend below 16,000 feet, but a greensward of vegetation creeps up to their bases, dwarf rhododendrons cover the moraines, and herbs grow on the patches of earth carried down by the latter, which are thawed by the more frequent sunshine, and by the radiation of heat from the unsnowed flanks of the valleys down which these ice-streams pour.
Looking eastward or westward on the map of India, we perceive that the phenomenon of perpetual snow is regulated by the same laws.
From the longitude of Upper a.s.sam in 95 degrees E to that of Kashmir in 75 degrees E, the lowest limit of perpetual snow is 15,500 to 16,000 feet, and a shrubby vegetation affects the most humid localities near it, at 12,000 to 14,000 feet. Receding from the plains of India and penetrating the mountains, the climate becomes drier, the snowline rises, and vegetation diminishes, whether the elevation of the land increases or decreases; plants reaching 17,000 and 18,000 feet, and the snow-line, 20,000 feet. To mention extreme cases; the snow-level of Sikkim in 27 degrees 30 minutes is at 16,000 feet, whereas in lat.i.tude 35 degrees 30 minutes Dr. Thomson found the snow line 20,000 feet on the mountains near the Karakoram Pa.s.s, and vegetation up to 18,500 feet--features I found to be common also to Sikkim in lat.i.tude 28 degrees.
The Himalaya, north of Nepal, and thence eastward to the bend of the Yaru-Tsampu (or Tibetan Burrampooter) has for its geographical limits the plains of India to the south, and the bed of the Yaru to the north. All between these limits is a mountain ma.s.s, to which Tibet (though so often erroneously called a plain)* [The only true account of the general features of eastern Tibet is to be found in MM. Huc and Gabet"s travels. Their description agrees with Dr. Thomson"s account of western Tibet, and with my experience of the parts to the north of Sikkim, and the information I everywhere obtained.
The so-called _plains_ are the flat floors of the valleys, and the terraces on the margins of the rivers, which all flow between stupendous mountains. The term "maidan," so often applied to Tibet by the natives, implies, not a plain like that of India, but simply an open, dry, treeless country, in contrast to the densely wooded wet regions of the snowy Himalaya, south of Tibet.] forms no exception.
The waters from the north side of this chain flow into the Tsampu, and those from the south side into the Burrampooter of a.s.sam, and the Ganges. The line, however tortuous, dividing the heads of these waters, is the watershed, and the only guide we have to the axis of the Himalaya. This has never been crossed by Europeans, except by Captain Turner"s emba.s.sy in 1798, and Captain Bogle"s in 1779, both of which reached the Yaru river. In the account published by Captain Turner, the summit of the watershed is not rigorously defined, and the boundary, of Tibet and Bhotan is sometimes erroneously taken for it; the boundary being at that point a southern spur of Chumulari.*
[Between Donkia and Chumulari lies a portion of Tibet (including the upper part of the course of the Machoo river) bounded on the east by Bhotan, and on the west by Sikkim (see chapter xxii). Turner, when crossing the Simonang Pa.s.s, descended westwards into the valley of the Machoo, and was still on the Indian watershed.] Eastwards from the sources of the Tsampu, the watershed of the Himalaya seems to follow a very winding course, and to be everywhere to the north of the snowy peaks seen from the plains of India. It is by a line through these snowy peaks that the axis of the Himalaya is represented in all our maps; because they _seem_ from the plains to be situated on an east and west ridge, instead of being placed on subsidiary meridional ridges, as explained above. It is also across or along the subsidiary ridges that the boundary line between the Tibetan provinces and those of Nepal, Sikkim, and Bhotan, is usually drawn; because the enormous acc.u.mulations of snow form a more efficient natural barrier than the greater height of the less snowed central part of the chain beyond them.
Though, however, our maps draw the axis through the snowy peaks, they also make the rivers to rise beyond the latter, on the northern slopes as it were, and to flow southwards through gaps in the axis.
Such a feature is only reconcilable with the hypothesis of the chain being double, as the Cordillera of Peru and Chili is said to be, geographically, and which in a geological sense it no doubt is: but to the Cordillera the Himalaya offers no parallel. The results of Dr. Thomson"s study of the north-west Himalaya and Tibet, and my own of the north-east extreme of Sikkim and Tibet, first gave me an insight into the true structure of this chain. Donkia mountain is the culminant point of an immensely elevated ma.s.s of mountains, of greater mean height than a similarly extensive area around Kinchin junga. It comprises Chumulari, and many other mountains much above 20,000 feet, though none equalling Kinchinjunga, Junnoo, and Kubra. The great lakes of Ramchoo and Cholamoo are placed on it; and the rivers rising on it flow in various directions; the Painomchoo north-west into the Yaru; the Arun west to Nepal; the Teesta south- west through Sikkim; the Machoo south, and the Pachoo south-east, through Bhotan. All these rivers have their sources far beyond the great snowed mountains, the Arun most conspicuously of all, flowing completely at the back or north of Kinchinjunga. Those that flow southwards, break through no chain, nor do they meet any contraction as they pa.s.s the snowy parts of the mountains which bound the valleys in which they flow, but are bound by uniform ranges of lofty mountains, which become more snowy as they approach the plains of India. These valleys, however, gradually contract as they descend, being less open in Sikkim and Nepal than in Tibet, though there bounded by rugged mountains, which from being so bare of snow and of vegetation, do not give the same impression of height as the isolated sharper peaks which rise out of a dense forest, and on which the snow limit is 4,000 or 5,000 feet lower.
The fact of the bottom of the river valleys being flatter towards the watershed, is connected with that of their fall being less rapid at that part of their course; this is the consequence of the great extent in breadth of the most elevated portion of the chain. If we select the Teesta as an example, and measure its fall at three points of its course, we shall find the results very different. From its princ.i.p.al source at Lake Cholamoo, it descends from 17,000 to 15,000 feet, with a fall of 60 feet to the mile; from 15,000 to 12,000 feet, the fall is 140 feet to the mile; in the third part of its course it descends from 12,000 to 5000 feet, with a fall of 160 feet to the mile; and in the lower part the descent is from 5000 feet to the plains of India at 300 feet, giving a fall of 50 feet to the mile.
There is, however, no marked limit to these divisions; its valley.
gradually contracts, and its course gradually becomes more rapid.
It is worthy of notice that the fall is at its maximum through that part of its valley of which the flanks are the most loaded with snow; where the old moraines are very conspicuous, and where the present acc.u.mulations from landslips, etc., are the most extensive.* [It is not my intention to discuss here the geological bearings of this curious question; but I may state that as the humidity of the climate of the middle region of the river-course tends to increase the fall in a given s.p.a.ce, so I believe the dryness of the climate of the loftier country has the opposite effect, by preserving those acc.u.mulations which have raised the floors of the valleys and rendered them level.]
With reference to Kinchinjunga, these facts are of importance, as showing that mere elevation is in physical geography of secondary importance. That lofty mountain rises from a spur of the great range of Donkia, and is quite removed from the watershed or axis of the Himalaya, the rivers which drain its northern and southern flanks alike flowing to the Ganges. Were the Himalaya to be depressed 18,000 feet, Kubra, Junnoo, Pundim, etc., would form a small cl.u.s.ter of rocky islands 1000 to 7000 feet high, grouped near Kinchinjunga, itself a cape 10,000 feet high, which would be connected by a low, marrow neck, with an extensive and mountainous tract of land to its north-east; the latter being represented by Donkia. To the north of Kinchin a deep bay or inlet would occupy the present valley of the Arun, and would be bounded on the north by the axis of the Himalaya, which would form a continuous tract of land beyond it. Since writing the above, I have seen Professor J. Forbes"s beautiful work on the glaciers of Norway: it fully justifies a comparison of the Himalaya to Norway, which has long been a familiar subject of theoretical enquiry with Dr. Thomson and myself. The deep narrow valleys of Sikkim admirably represent the Norwegian fiords; the lofty, rugged, snowy mountains, those more or less submerged islands of the Norwegian coast; the broad rearward watershed, or axis of the chain, with its lakes, is the same in both, and the Yaru-tsampu occupies the relative position of the Baltic.
Along the whole chain of the Himalaya east of k.u.maon there are, I have no doubt, a succession of such lofty ma.s.ses as Donkia, giving off stupendous spurs such as that on which Kinchin forms so conspicuous a feature. In support of this view we find every river rising far beyond the snowy peaks, which are separated by continuously unsnowed ranges placed between the great white ma.s.ses that these spurs present to the observer from the south.* [At vol. i.
chapter viii, I have particularly called attention to the fact, that west of Kinchinjunga there is no continuation of a snowy Himalaya, as it is commonly called. So between Donkia and Chumulari there is no perpetual snow, and the valley of the Machoo is very broad, open, and comparatively flat.] From the Khasia mountains (south-east of Sikkim) many of these groups or spurs were seen by Dr. Thomson and myself, at various distances (80 to 210 miles); and these groups were between the courses of the great rivers the Soobansiri, Mona.s.s, and Pachoo, all east of Sikkim. Other ma.s.ses seen from the Gangetic valley probably thus mark the relative positions of the Arun, Cosi, Gunduk, and Gogra rivers.
Another ma.s.s like that of Chumulari and Donkia, is that around the Mansarowar lakes, so ably surveyed by the brothers Captains R. and H.
Strachey, which is evidently the centre of the Himalaya. From it the Gogra, Sutlej, Indus, and Yaru rivers all flow to the Indian side of Asia; and from it spring four chains, two of which are better known than the others. These are:--1. The eastern Himalaya, whose axis runs north of Nepal, Sikkim, and Bhotan, to the bend of the Yaru, the valley of which it divides from the plains of India. 2. The north-west Himalaya, which separates the valley of the Indus from the plains of India. Behind these, and probably parallel to them, lie two other chains. 3. The Kouenlun or Karakoram chain, dividing the Indus from the Yarkand river. 4. The chain north of the Yaru, of which nothing is known. All the waters from the two first of these chains, flow into the Indian Ocean, as do those from the south faces of the third and fourth; those from the north side of the Kouenlun, and of the chain north of the Yaru, flow into the great valley of Lake Lhop, which may once have been continuous with the Amoor river.* [The Chinese a.s.sert that Lake Lhop once drained into the Hoang-ho; the statement is curious, and capable of confirmation when central Asia shall have been explored.]
For this view of the physical geography of the western Himalaya and central Asia, I am indebted to Dr. Thomson. It is more consonant with nature, and with what we know of the geography of the country and of the nature of mountain chains, than that of the ill.u.s.trious Humboldt, who divides central Asia by four parallel chains, united by two meridional ones; one at each extremity of the mountain district.
It follows in continuation and conclusion of our view that the mountain ma.s.s of Pamir or Bolor, between the sources of the Oxus and those of the Yarkand river, may be regarded as a centre from which spring the three greatest mountain systems of Asia. These are:--1. A great chain, which runs in a north-easterly direction as far as Behring"s Straits, separating all the rivers of Siberia from those which flow into the Pacific Ocean. 2. The Hindoo Koosh, continued through Persia, and Armenia into Taurus. And, 3. The Muztagh or Karakorum, which probably extends due east into China, south of the Hoang-ho, but which is broken up north of Mansarowar into the chains which have been already enumerated.
APPENDIX F.
ON THE CLIMATE OF SIKKIM.
The meteorology of Sikkim, as of every part of the Himalayan range, is a subject of growing interest and importance; as it becomes yearly more necessary for the Government to afford increased facilities for a residence in the mountains to Europeans in search of health, or of a salubrious climate for their families, or for themselves on retirement from the exhausting service of the plains. I was therefore surprised to find no further register of the weather at Dorjiling, than an insufficient one of the rain-fall, kept by the medical officer in charge of the station; who, in this, as in all similar cases,* [The government of India has gone to an immense expense, and entailed a heavy duty upon its stationary medical officers, in supplying them with sometimes admirable, but more often very inaccurate, meteorological instruments, and requiring that daily registers be made, and transmitted to Calcutta. In no case have I found it to be in the officer"s power to carry out this object; he has never time, seldom the necessary knowledge and experience, and far too often no inclination. The majority of the observations are in most cases left to personal native or other servants, and the laborious results I have examined are too frequently worthless.] has neither the time nor the opportunity to give even the minimum of required attention to the subject of meteorology. This defect has been in a measure remedied by Dr. Chapman, who kept a twelve-months"
register in 1837, with instruments carefully compared with Calcutta standards by the late James Prinsep, Esq., one of the most accomplished men in literature and science that India ever saw.
The annual means of temperature, rain-fall, etc., vary greatly in the Himalaya; and apparently slight local causes produce such great differences of temperature and humidity, that one year"s observations taken at one spot, however full and accurate they may be, are insufficient: this is remarkably the case in Sikkim, where the rainfall is great, and where the difference between those of two consecutive years is often greater than the whole annual London fall.
My own meteorological observations necessarily form but a broken series, but they were made with the best instruments, and with a view to obtaining results that should be comparable _inter se,_ and with those of Calcutta; when away from Dorjiling too, in the interior of Sikkim, I had the advantage of Mr. Muller"s services in taking observations at hours agreed upon previous to my leaving, and these were of the greatest importance, both for calculating elevations, and for ascertaining the differences of temperature, humidity, diurnal atmospheric tide, and rain-fall; all of which vary with the elevation, and the distance from the plains of India.
Mr. Hodgson"s house proved a most favourable spot for an observatory, being placed on the top of the Dorjiling spur, with its broad verandah facing the north, in which I protected the instruments from radiation* [This is a most important point, generally wholly neglected in India, where I have usually seen the thermometer hung in good shade, but exposed to reflected heat from walls, gravel walks, or dry earth. I am accustomed from experience to view all extreme temperatures with great suspicion, on this and other accounts. It is very seldom that the temperature of the free shaded air rises much above 100 degrees, except during hot winds, when the lower stratum only of atmosphere (often loaded with hot particles of sand), sweeps over the surface of a soil scorched by the direct rays of the sun.]
and wind. Broad gra.s.s-plots and a gravel walk surrounded the house, and large trees were scattered about; on three sides the ground sloped away, while to the north the spur gently rose behind.
Throughout the greater part of the year the prevailing wind is from the south-east, and comes laden with moisture from the Bay of Bengal: it rises at sunrise, and its vapours are early condensed on the forests of Sinchul; billowy clouds rapidly succeed small patches of vapour, which rolling over to the north side of the mountain, are carried north-west, over a broad intervening valley, to Dorjiling.
There they bank on the east side of the spur, and this being partially clear of wood, the acc.u.mulation is slow, and always first upon the clumps of trees. Very generally by 9 a.m., the whole eastern sky, from the top of Dorjiling ridge, is enveloped in a dense fog, while the whole western exposure enjoys sunshine for an hour or two later. At 7 or 8 a.m., very small patches are seen to collect on Tonglo, which gradually dilate and coalesce, but do not shroud the mountain for some hours, generally not before 11 a.m. or noon.
Before that time, however, ma.s.ses of mist have been rolling over Dorjiling ridge to the westward, and gradually filling up the valleys, so that by noon, or 1 p.m., every object is in cloud.
Towards sunset it falls calm, when the mist rises, first from Sinchul, or if a south-east wind sets in, from Tonglo first.
The temperature is more uniform at Mr. Hodgson"s bungalow, which is on the top of the Dorjiling ridge, than on either of its flanks; this is very much because a good deal of wood is left upon it, whose cool foliage attracts and condenses the mists. Its mean temperature is lower by nearly 2.5 degrees than that of Mr. Muller"s and Dr.
Campbell"s houses, both situated on the slopes, 400 feet below.
This I ascertained by numerous comparative observations of the temperature of the air, and by burying thermometers in the earth: it is chiefly to be accounted for by the more frequent sunshine at the lower stations, the power of the sun often raising the thermometer in shade to 80 degrees, at Mr. Muller"s; whereas during the summer I spent at Mr. Hodgson"s it never rose much above 70 degrees, attaining that height very seldom and for a very short period only. The nights, again, are uniformly and equally cloudy at both stations, so that there is no corresponding cold of nocturnal radiation to reduce the temperature.
The mean decrease of temperature due to elevation, I have stated (Appendix I.) to be about 1 degree for every 300 feet of ascent; according to which law Mr. Hodgson"s should not be more than 1.5 degrees colder than Mr. Muller"s. These facts prove how difficult it is to choose unexceptionable sites for meteorological observatories in mountainous countries; discrepancies of so great an amount being due to local causes, which, as in this case, are perhaps transient; for should the top of the spur be wholly cleared of timber, its temperature would be materially raised; at the expense, probably, of a deficiency of water at certain seasons. Great inequalities of temperature are also produced by ascending currents of heated air from the Great Rungeet valley, which affect certain parts of the station only; and these raise the thermometer 10 degrees (even when the sun is clouded) above what it indicates at other places of equal elevation.
The mean temperature of Dorjiling (elev. 7,430 feet) is very nearly 50 degrees, or 2 degrees higher than that of London, and 26 degrees below that of Calcutta (78 degrees,* [Prinsep, in As. Soc. Journ., Jan. 1832, p. 30.] or 78.5 degrees in the latest published tables*
[Daniell"s Met. Essays, vol. ii. p. 341.]); which, allowing 1 degree of diminution of temperature for every degree of lat.i.tude leaves 1 degree due to every 300 feet of ascent above Calcutta to the height of Dorjiling, agreeably to my own observations. This diminution is not the same for greater heights, as I shall have occasion to show in a separate chapter of this Appendix, on the decrement of heat with elevation.
A remarkable uniformity of temperature prevails throughout the year at Dorjiling, there being only 22 degrees difference between the mean temperatures of the hottest and coldest months; whilst in London, with a lower mean temperature, the equivalent difference is 27 degrees. At 11,000 feet this difference is equal to that of London.
In more elevated regions, it is still greater, the climate becoming excessive at 15,000 feet, where the difference amounts to 30 degrees at least.* [This is contrary to the conclusions of all meteorologists who have studied the climate of the Alps, and is entirely due to the local disturbances which I have so often dwelt upon, and princ.i.p.ally to the unequal distribution of moisture in the loftier rearward regions, and the aridity of Tibet. Professor James Forbes states (Ed.
Phil. Trans., v. xiv. p. 489):--1. That the decrement of temperature with alt.i.tude is most rapid in summer: this (as I shall hereafter show) is not the case in the Himalaya, chiefly because the warm south moist wind then prevails. 2. That the annual range of temperature diminishes with the elevation: this, too, is not the case in Sikkim, because of the barer surface and more cloudless skies of the rearward loftier regions. 3. That the diurnal range of temperature diminishes with the height: that this is not the cane follows from the same cause. 4. That radiation is least in winter: this is negatived by the influence of the summer rains.] The accompanying table is the result of an attempt to approximate to the mean temperatures and ranges of the thermometer at various elevations.
Alt.i.tude 11,000 feet 15,000 feet 19,000 feet Mean shade 40.9 29.8 19.8 Mean warmest month 50.0 40.0 32.0 Mean coldest month 24.0 11.0 0.0 Mean daily range of temperature 20.0 27.0 35.0 Rain-fall in inches 40.0 20.0 10.0 1 degree equals 320 feet 350 feet 400 feet
Supposing the same formula to apply (which I exceedingly doubt) to heights above 19,000 feet, 2 degrees would be the mean annual temperature of the summit of Kinchinjunga, alt.i.tude 28,178 feet, the loftiest known spot on the globe: this is a degree or two higher than the temperature of the poles of greatest cold on the earth"s surface, and about the temperature of Spitzbergen and Melville island.
The upper limit of phenogamic vegetation coincides with a mean temperature of 30 degrees on the south flank of Kinchinjunga, and of 22 degrees in Tibet; in both cases annuals and perennial-rooted herbaceous plants are to be found at elevations corresponding to these mean temperatures, and often at higher elevations in sheltered localities. I have a.s.sumed the decrease of temperature for a corresponding amount of elevation to be gradually less in ascending (1 degree=320 feet at 6000 to 10,000 feet, 1 degree=400 feet at 14,000 to 18,000 feet). My observations appear to prove this, but I do not regard them as conclusive; supposing them to be so, I attribute it to a combination of various causes, especially to the increased elevation and yet unsnowed condition of the ma.s.s of land elevated above 16,000 feet, and consequent radiation of heat; also to the greater amount of sunshine there; and to the less dense mists which obstruct the sun"s rays at all elevations. In corroboration of this I may mention that the decrease of temperature with elevation is much less in summer than in winter, 1 degree of Fahr. being equivalent to only 250 feet in January between 7000 and 13,000 feet, and to upwards of 400 feet in July. Again, at Dorjiling (7,430 feet) the temperature hardly ever rises above 70 degrees in the summer months, yet it often rises even higher in Tibet at 12,000 to 14,000 feet. On the other hand, the winters, and the winter nights especially, are disproportionately cold at great heights, the thermometer falling upwards of 40 degrees below the Dorjiling temperature at an elevation only 6000 feet higher.
The diurnal distribution of temperature is equally and similarly affected by the presence of vapour at different alt.i.tudes. The lower and outer ranges of 6000 to 10,000 feet, first receive the diurnal charge of vapour-loaded southerly winds; those beyond them get more of the sun"s rays, and the rearward ones more still. Though the summer days of the northern localities are warmer than their elevation would indicate, the nights are not proportionally cold; for the light mist of 14,000 feet, which replaces the dense fog of 7000 feet, effectually obstructs nocturnal radiation, though it is less an obstacle to solar radiation. Clear nights, be it observed, are as rare at Momay (15,300 feet) as at Dorjiling, the nights if windy being rainy; or, if calm, cold currents descend from the mountains, condensing the moist vapours of the valleys, whose narrow floors are at sunrise bathed in mist at all elevations in Sikkim. The rise and dispersion of these dense mists, and their collection and recondensation on the mountains in the morning, is one of the most magnificent phenomena of the Himalaya, when viewed from a proper elevation; it commences as soon as the sun appears on the horizon.
The mean daily range of the thermometer at 7000 feet is 13 degrees in cleared spots, but considerably less in wooded, and certainly one-third less in the forest itself. At Calcutta, which has almost an insular climate, it amounts to 17 degrees; at Delhi, which has a continental one, to 24.6 degrees; and in London to 17.5 degrees.
At 11,000 feet it amounts to about 20 degrees, and at 15,000 feet to 27 degrees. These values vary widely in the different months, being much less in the summer or rainy months. The following is probably a fair approximation:--
At 7,000 feet it amounts to 8-9 degrees in Aug. and Sept., and 17 degrees in Dec. At 11,000 feet it amounts to 12 degrees in Aug. and Sept., and 30 degrees in Dec. At 15,000 feet it amounts to 15 degrees in Aug. and Sept., and 40 degrees in Dec. At London it amounts to 20 degrees in Aug. and Sept., and 10 degrees in Dec.
The distribution of temperature throughout the day and year varies less at Dorjiling than in most mountainous countries, owing to the prevailing moisture, the effect of which is a.n.a.logous to that of a circ.u.mambient ocean to an island: the difference being, that in the case of the island the bulk of water maintains an uniform temperature; in that of Dorjiling the quant.i.ty of vapour acts directly by interfering with terrestrial and solar-radiation, and indirectly by nurturing a luxuriant vegetation. The result in the latter case is a climate remarkable for its equability, and similar in many features to that of New Zealand, South-west Chili, Fuegia, and the damp west coasts of Scotland and Ireland, and other countries exposed to moist sea winds.
The mean temperature of the year at Dorjiling, as taken by maxima and minima thermometers* [The mean of several of the months, thus deduced, often varies a good deal from the truth, owing to the unequal diurnal distribution of heat; a very few minutes" sunshine raises the temperature l0 degrees or 15 degrees above the mean of the day; which excessive heat (usually transient) the maximum thermometer registers, and consequently gives too high a mean.] by Dr. Chapman, is nearly the same as that of March and October: January, the coldest month, is more than 13.4 degrees colder than the mean of the year; but the hottest month is only 8.3 degrees warmer than the same mean: at Calcutta the months vary less from the mean; at Delhi more; and in London the distribution is wholly different; there being no rains to modify the summer heat, July is 13 degrees hotter, and January 14 degrees colder than the mean of the year.
This distribution of the seasons has a most important effect upon vegetation, to which sufficient attention has not been paid by cultivators of alpine Indian plants; in the first place, though English winters are cold enough for such, the summers are too hot and dry; and, in the second place, the great accession of temperature, causing the buds to burst in spring, occurs in the Himalaya in March, when the temperature at 7000 feet rises 8 degrees above that of February, raising the radiating thermometer always above the freezing point, whence the young leaves are never injured by night frost: in England the corresponding rise is only 3 degrees, and there is no such accession of temperature till May, which is 8 degrees warmer than April; hence, the young foliage of many Himalayan plants is cut off by night frosts in English gardens early in the season, of which _Abies Webbiana_ is a conspicuous example.
The greatest heat of the day occurs at Dorjiling about noon, owing to the prevalent cloud, especially during the rainy months, when the sun shines only in the mornings, if at all, and the clouds acc.u.mulate as the day advances. According to hourly observations of my own, it occurred in July at noon, in August at 1 p.m., and in September (the most rainy month) there was only four-tenths of a degree difference between the means of noon, 1 p.m., and 2 p.m., but I must refer to the abstracts at the end of this chapter for evidence of this, and of the wonderful uniformity of temperature during the rainy months.
In the drier season again, after September, the greatest heat occurs between 2 and 3 p.m.; in Calcutta the hottest hour is about 2.45 p.m., throughout the year; and in England also about 3 p.m.