63.) 



SNOW, PERENNIAL OR PERPETUAL. 



SNOW, PERENNIAL OR PERPETUAL. 



634 



lowest temperatures. The extraordinary complexity of many of these 

 crystals of snow; when considered in connection with the principle of 

 regelation which governs the union of separate portions of ice, leads 

 to the suspicion that infinitesimally small crystals result from the 

 solidification of vapour, which undergo accretion and are subse- 

 quently incorporated with each other by regelation, in directions 

 accordant with those of the crystalline forces to which the elementary 

 forms are due. The crystallography, however, of solid water, or ice, 

 which of course is that of snow, is at present in a condition of singular 

 contradiction. We appear not to know with certainty to what system 

 of crystallisation it should be referred, or even whether it does not 

 belong to more than one system ; though there is strong evidence that 

 it must belong, in part at least, to the rhombohedral system, to which 

 it has usually been referred. The facts known respecting it will be 

 stated with the other properties of ice, in the article WATER. 



Snow, in the form of cylinders and spheres or spheroids, has been 

 occasionally observed in North America. The former were produced by 

 the snow deposited in a second shower upon some which had pre- 

 viously fallen, and the surface of which had been covered by a thin 

 coating of ice. A violent wind then caused the particles of snow to 

 roll on the ice, and the masses thus produced assumed perfectly 

 cylindrical forms of various sizes, the greatest being 2J or 3 feet 

 diameter ; they were hollow at each end. The spherical balls were 

 from 1 inch to 15 inches in diameter, and were also formed chiefly by 

 rolling, though some were found in enclosures where they could not have 

 rolled, and, therefore, they are supposed to have been formed in the 

 atmosphere itself ; they were very light, and were composed of crystals 

 irregularly united. (Sillinian's ' Journal,' vols. ii. and vi.) Similar 

 balls were observed in East Lothian, in 1830, by Mr. Sheriff; and this 

 gentleman relates that they were composed only of snow, for one of 

 them being cut through was found to have no hard body for its 

 nucleus. ( Edin. Phil. Journal.,' ii. 58.) Mr. Luke Howard records 

 an instance, in which, with the surface at 33" or 31, and during a strong 

 wind, the snow, instead of driving loose before the wind, was collected 

 occasionally into a ball, which rolled on, increasing till its weight 

 stopped it : thousands of such balls were seen lying in the fields, some 

 of them several inches in diameter. The balls of snow, torrents of 

 which constitute what are called rolling avalanches [AVALANCHES], 

 appear to be formed in a similar manner, though by means chiefly 

 mechanical, in addition to regelation, which must have place in all these 

 accretion.-. 



When an extensive tract of country is covered with newly-fallen 

 now, its glare has a painful and injurious effect on the eyes, from 

 which the traveller has to guard them by a crape veil, the natives 

 using various similar means of protection. It is, however, from such 

 snow alone that much inconvenience is felt, indicating, probably, that 

 it is owing to the light reflected from the myriads of facets which the 

 crystals of snow present. Dr. Joseph D. Hooker remarks, that he has 

 never suffered inconvenience from this cause in crossing beds of old 

 snow, or glaciers with weathered surfaces, which absorb a great deal of 

 light, and reflect comparatively little, and that little coloured green or 

 blue. The changes to which snow is subject after descending to the 

 ground, according to the circumstances of temperature, weather, ex- 

 posure, and the nature of the surface upon which it has fallen, are 

 evaporation, liquefaction, and conversion into a compact ice. It evapo- 

 rate* at all temperatures. Mr. Howard found a circular area, of five 

 inches diameter, to lose 150 grains troy from sunset to sunrise, and 

 about 50 grains more by the following sunset, the temperature varying 

 from 18 to 30. This evaporation probably supplies the vapour which 

 appears in the form of mist after a snow-fall, and also that which is 

 condensed again in the form of a secondary fall of snow. 



The process of conversion of snow into glacier ice has been noticed 

 in the article GLACIERS, in NAT. HIST. Drv. A similar conversion, in 

 localities of a different character, has been observed by Dr. Hooker, 

 who describes the beds of perennial snow in the Sikkim Himalaya, 

 which extend below the true inferior limit of perpetual snow (see the 

 following article), as having great resemblance to glaciers, from which, 

 indeed, he considers them undistinguishable. Though broad and con- 

 vex, and occupying mountain slopes, not filling hollows like glaciers 

 > oniinonly so called, they display the ribboned structure of glacier- 

 ice, and descend at a rate and to a distance depending on the slope 

 and on the amount of annual accumulation behind. [CLOUD : HAIL.! 



SNOW, PERENNIAL, or PEItPETUAL. In the article CLIMATK 

 (cot 968-70), a general view of this subject has been given, 

 together with particulars of the elevation of the snow-line under 

 different latitudes and in different localities, both as indicated by theory 

 and shown by observation. We are now enabled to enter in a more 

 precise manner into the consideration of the varying circumstances 

 which govern the position of the snow-line, and also into the special 

 history of perpetual snow as it exist* about and upon the greatest and 

 highest moTintains of the globe. For the materials of these we are 

 chiefly indebted to Dr. Joseph D. Hooker, who, in the Appendix to his 

 ' Himalayan Journals,' has treated the subject with views at once more 

 general and more philosophical, perhaps, than have been applied to it 

 since the original investigations of Wahlenberg, Von Buch, and 

 Htiiulioldt ; and has largely illustrated them by the facts of the dis- 

 tribution of snow in Nepaul, Tibet, and the Himalaya, which are 

 stated in their appropriate places in his 'Journals ' themselves. 



It must be premised that three things are frequently confounded 

 together m popular science and in books of travels. First, the 

 elevation in the atmosphere, or above the general surface of the earth, 

 as defined by the level of the sea, of the temperature of 32 Fahr., or 

 the freezing point, for each latitude, as affected by the distribution of 

 land and sea, from which, in union with the solar temperature, results 

 the temperature of each isothermal line at the surface. 



Secondly, the position of the inferior limit of the beds of perpetual 

 snow the actual snow-line in this sense on mountainous elevations, 

 as dependent on the cause just assigned, in conjunction with local 

 causes on the great scale, and as existing in fact at that particular 

 elevation for each region, below which all the snow that falls melts. 

 This may be considerably below or considerably above the line of 32. 



Thirdly, the altitude at which much snow is perennial, in the 

 ordinary sense of the word its permanence arising from local and 

 from temporary (though recurrent) meteorological causes masking, 

 by its extension downwards, the true limit of perpetual snow as 

 defined above. It is requisite, however, for the due apprehension of 

 the subject, that the distinction between these three things should be 

 carefully preserved in the mind. 



Dr. Hooker has stated his belief (' Him. Journ.,' vol. ii., p. 394), 

 " that the limit of perpetual snow is laid down too low in all moun- 

 tain regions, and that accumulations in hollows, and the descent of 

 glacial ice, mask the phenomenon more effectually than is generally 

 allowed." He defines the limit, " in general terms only, as being that 

 where the accumulations are very great, and whence they are con- 

 tinuous upwards, on gentle slopes. All perpetual snow, however," he 

 continues, " becomes ice, and, as such, obeys the laws of glacial motion 



; whence it follows, that the lower edge of a suow-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. ... It is im- 

 possible 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 per- 

 petual snow." A careful examination of those great beds of snow in 

 the Alps, from whose position the mean lower level of perpetual snow, 

 in that latitude, has been deduced, convinced Dr. Hooker that they 

 are winter accumulations of the kind alluded to above, due mainly 

 to eddies of wind of far more snow than can be melted in the follow- 

 ing summer, being hence perennial in the ordinary sense of the word 

 only. It follows that the true limit of perpetual snow is much higher 

 in the Alps than it is usually supposed to be. He proceeds to show 

 that the altitude of the limit in the Alps has been stated by Professor 

 James Forbes more than 1000 feet in defect, the Jardin, on the Mer de 

 Glace, at the elevation of 9500 feet, being evidently below the limit, 

 to which, however, Professor Forbes had assigned the elevation o 

 8500 feet only. 



Proceeding to the principal scene of Dr. Hooker's own researches, 

 we find that there are two secondary considerations which materially 

 affect the melting of snow, and therefore exert a material influence on 

 the elevation of the snow-line, but which have not been sufficiently 

 dwelt upon, though they bear directly upon the great altitude of that 

 line in the most elevated regions. From the imperfect transmission of 

 the heating rays of the sun through films of water, it follows that the 

 direct effects of the rays, in clear sunshine, are very different at equal 

 elevations of the moist outer and dry inner ranges of the Himalaya. 

 Secondly, naked rock and soil absorb much more heat than 

 surfaces covered with vegetation, and this heat, radiated from them 

 again, is much more rapidly absorbed by the white snow than the 

 direct heat of the sun's rays in. Hence, at equal elevations, the ground 

 heats sooner, and the snow is more exposed to the heat thus radiated 

 in arid Tibet than in the wooded and grassed mountains of Sikkitu. 

 In the latter region, "the position and elevation of the perpetual snow 

 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 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 little snow on it, most of what there is 

 lying upon north exposures." Thus appears to be at last explained the 

 apparent anomaly that the snow-line ascends in advancing north to 

 the coldest Himalayan region; the position of the greatest peaks 

 and of the greatest mass of perpetual snow being generally assumed, 

 though erroneously, 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 assertion, that the 

 snow lies longer and deeper on the north than on the south slope of 

 the Himalayan ridge." 



The enormous accumulation of snow in Sikkim at 15,000 feet 

 exercises a decided influence on the vegetation, preventing its exten- 

 sion upwards, which, in other situations, takes place to 16,000 and 

 17,000 feet. Glaciers 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 



