Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/869

HIMALAYA 829 iteor- ect of iUU- ns on no- icre. ro- tric sssure a known origin can be assigned. The contraction of the cooling but now solid crust of the earth must have set up great horizontal strains, partly of tension and partly of compression, which would necessarily have been followed by rupture or crushing along lines of least resistance, and the movements on such lines are marked by the great mountain ranges that traverse the surface. A dislocation of the solid crust of the earth once having taken place, it would probably continue to be a line of least resistance ever after, and a succession of movements during past geological periods may thus be reasonably expected along such lines. Somewhat in proportion as the disturbing forces are intense, and the thickness of the crust on which they act is great, will be the tendency for the lines of rupture to be continuous for a considerable distance ; and as the disturbed area is extended in its dimensions, the probability will increase of a repetition of a series of similar dislocations on lines approximately parallel to, or at right angles to, one another, and to the line on which the greatest compression and consequent tension tuke place. In a disturbed area., one transverse dimension of which is sensibly greater than the rest, the longitudinal ruptures will predominate in the interior and the transverse towards the borders. Almost all mountains give indica tions of having been shaped by forces thus related, and to the action of such forces may the main characteristics of the structure of the Himalaya, and the arrangement of its ridges and v.illeys, be attributed. Whatever may be the power of rivers in general as instruments of erosion, and whatever effect the Himalayan rivers have had in removing the fragments of the rocks over and among which they look their courses, it is hardly possible to doubt that their main directions were determined by the anterior lines of dislocation which opened up hollows down which they could tlow, and which must invariably have been accom panied by a destructive and crushing action on the rocks along them, which has enabled the waters the more readily to sweep away the obstacles in their path. The parallelism of many of the great Tibetan and Himalayan rivers for hundreds of miles together, amid such mountains, seems wholly inexplicable in any other manner. Although the loftiest mountains when compared to the earth s diameter are insignificant in their dimensions, and the irregularities of the surface would hardly be perceptible on any sphere, however large, that could be made to represent the earth, yet in relation to the depth of the atmosphere even moderate elevations become of great im portance, and such heights as those reached by the Himalaya introduce modifications of climate in ascending over its slopes that are not surpassed by those observed in moving from the equator to the poles. One half of the total mass of the atmosphere and three-fourths of the water suspended in it in the form of vapour lie below the average elevation of the Himalaya, and of the residue one half of the air and virtually almost all the vapour come within the influence of the highest peaks. The general changes of pressure of the atmosphere indicated by the barometer extend in a modified but well-marked manner to the greatest elevations to which observation has been carried, and the annual and diurnal oscillations are not less regular in Tibet than in the plains of India, though somewhat reduced in amount. With the increase of elevation the diminution of the quantity of vapour held in suspension is very marked, and at the greatest heights reached there is found not more than one- sixth or one-seventh part of that observed at the foot of the mountains, and the proportion is sometimes as low as a tenth, or less. As is well known, the maximum quantity of water that can be suspended in the air in a state of vapour depends on the temperature of the air, and observa tion has established that the actual quantity at all eleva tions is approximately proportionate to this maximum, and is thus determined by the temperature, which in turn is regulated by the elevation. The theoretical view onco held that watery vapour was distributed in the atmosphere in accordance with the laws of pressure of elastic fluids is manifestly inconsistent with observed facts, which indicate that the diffusion of vapour is powerfully obstructed by the air particles, and that it by no means behaves as an independent elastic fluid, but rather as though it were merely mixed up with or entangled among the air particles. The great elevations to which the Himalaya ascends, Tempei and the broad zone which it covers in respect to latitude, ature c and its varying distance from the sea necessarily lead to Hima- correspondingly great variations of climate in its different aya parts, including the temperature and the degree of humidity in the atmosphere, and the amount of rainfall. The general position of the whole tract between the 25th arid 35lh degrees of north latitude renders it subject to high summer temperatures. The heat of the great plain of northern India is not surpassed at any other part of the earth, the air temperature rising in the hottest months to more than 110 Fahr. ; the fall of temperature in the winter is . considerable, the thermometer receding at times to the freezing point, or even a little below it ; the mean tem perature varies from 78 Fahr. in the east or southern portion to 75 in the west or north. On the mountains every altitude has its corresponding temperature, an eleva tion of 1000 feet producing a fall of about 3^, or 1 Fahr. to 300 feet. The mean winter temperature at 7000 j feet is about 44 Fahr., with a mean minimum of 32 Fahr., and the summer mean about 65 Fahr. At 9000 feet the mean temperature of the coldest month is 32" Fahr., at 12,000 feet the thermometer ceases to fall below freezing point from the end of May to the middle of October, and at 15,000 feet it is seldom above that point at the coldest part of the day in the height of summer. There seems to be less variation of temperature at equal altitudes on the mountains in passing from east to west than might have been expected from the greater winter cold of the western part of the plains. This is pro bably due to the greater relative humidity and heavier rainfall of the eastern regions, and the denser vegetation which covers them. These influences tend to keep down the day temperature on the eastern mountains, while the clearer skies and more open character of the west operate in the opposite direction. On the more wocdy and rainy mountains in the outer parts of the chain the mean summer temperature will be 60 at 10,000 or 11,000 feet, but it will still be as much as that up to 12,000 feet, or even to a greater elevation, in the bare and sunny valleys near the Indian watershed. In Tibet the thermometrical conditions vary considerably Of Tib< from those of the Himalayan slope. At 12,000 feet the maximum temperature is perhaps 70 Fahr. and the absolute minimum 5 Fahr. The ordinary winter tempera ture is from Fahr. to 30 Fahr., with a mean of the coldest month of 10. The mean of the hottest month is about 60, and of the year 35. At 15,000 feet the frost is permanent from the end of October to the end of April, and the lakes are usually frozen over for nearly five months in the year. Between 15,000 and 16,000 feet the thermo meter will fall below the freezing point every night of the year. At heights of 17,000 or 18,000 feet it rises consider ably above that point in the summer months. From 18,000 to 19,000 feet it thaws only in the afternoons of July and August, and at 20,000 feet there is probably perpetual frost in the shade, though in the sun the air no doubt rises above

the freezing point to much greater elevations.