Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/757

] Malay peninsula. The same results are found along the mountains at a distance from the sea, the heaviest rainfall known to occur anywhere in the world (not less than 600 inches in the year) being recorded on the Khasiya range about 100 miles north-east of Calcutta, which presents an abrupt front to the progress of the moist winds flowing up from the Bay of Bengal. The cessation of the rains on the southern border of Baluchistan, west of Kurrachee, obviously arises from the projection of the south-east coast of Arabia, which limits the breadth of the S.W. monsoon air current and the length of the coast line directly exposed to it. The very small and irregular rainfall in Sindh and along the Indus is to be accounted for by the want of any obstacle in the path of the vapour-bearing winds, which, therefore, carry the uncondensed rain up to the Punjab, where it falls on the outer ranges of the western Himalaya and of Afghanistan.

65. Somewhat similar results, though on a smaller scale, attend the operation of the well-known land and sea breezes, which are universally prevalent in hot countries bordering on the sea. The relative greater heating of the land than of the sea during the day disturbs the planes of atmospheric equilibrium, and a dispersion of air in the higher regions from over the land leads to a diminution of pressure there and an increase over the sea. This causes the sea breeze, which is an inflow of moist air over the land from below ; and where, as is frequently the case, this breeze is forced, as it advances, to rise considerably above the sea level, condensation takes place on the mountain slopes either in clouds or rain. The constant precipitation of rain on tropical coasts is mainly due to this action.

66. An analogous, though less well understood, system of alternating winds is almost invariably set up over mountains rising abruptly from plains, currents blowing from the higher ground to the lower during the night, and from the lower to the higher during the day. Such winds are often combined with the land and sea breezes, which they tend to exaggerate. The diurnal mountain winds are very strongly marked on the Himalaya, where they probably are the most active agents in determining the precipitation of rain along the chain the monsoon currents, as was before stated, not penetrating among the mountains. The formation of dense banks of cloud in the afternoon, when the up wind is strongest, along the southern face of the snowy ranges of the Himalaya, is a regular daily phenomenon during the hotter months of the year, and heavy rain, accompanied by electrical discharges, is the frequent result of such condensation.

67. Too little is known of the greater part of Asia to admit of any more being said with reference to this part of the subject, than to mention a few facts bearing on the rainfall. At Tiflis the yearly fall is 22 inches ; on the Caspian about 7 or 8 inches ; on the Sea of Aral 5 or 6 inches. In South-western Siberia it is 12 or 14 inches, diminishing as we proceed eastward to 6 or 7 inches at Barnaul, and to 5 or 6 inches at Urga in Northern Mongolia. At Nertschinsk in Eastern Siberia it is about 15 to 20 inches. In China we find about 23 inches to be the fall at Peking ; while at Canton, which lies nearly on the northern tropic and the region of the S.W. monsoon is entered, the quantity is increased to 78 inches. At Batavia in Java the fall is about 78 inches ; at Singapore it is nearly 100 inches. The quantity increases considerably on that part of the coast of the Malay peninsula which is not sheltered from the south-west by Sumatra. On the Tenasserim and Burmese coast falls of more than 200 inches are registered, and the quantity is here nowhere less than 75 or 80 inches, which is about the average of the eastern part of the delta of the Ganges, Calcutta standing at about 6-4 inches. On the hills that flank Bengal on the east the fall is very great. On the Khasiya hills, at an elevation of about 4500 feet, the average of 10 years is more than 550 inches. As much as 150 inches has been measured in one month, and 610 inches in one year. On the west coast of the Indian peninsula the fall at the sea level varies from about 75 to 100 inches, and at certain elevations on the mountains more than 250 inches is commonly registered, with intermediate quantities at intervening localities. On the east coast the fall is far less, nowhere rising to 50 inches, and towards the southern apex of the peninsula being reduced to 25 or 30 inches. Ceylon shows from 60 to 80 inches. As we recede from the coast the fall diminishes, till it is reduced to about 25 or 30 inches at the head of the Gangetic plain. The tract along the Indus to within 60 or 80 miles of the Himalaya is almost rainless, 6 or 8 inches being the fall in the southern portion of the Punjab. On the outer ranges of the Himalaya the yearly fall amounts to about 200 inches on the east in Sikim, and gradually diminishes on the west, where north of the Pun jab it is about 70 or 80 inches. In the interior of the chain the rain is far less, and the quantity of precipitation is so small in Tibet that it can be hardly measured. It is to the greatly reduced fall of snow on the northern faces of the highest ranges of the Himalaya that is to be attributed the higher level of the snow-line, a phenomenon which was long a cause of discussion.

68. In Afghanistan, Persia, Asia Minor, and Syria, winter and spring appear to be the chief seasons of condensation. In other parts of Asia the principal part of the rain falls between May and September, that is, in the hottest half of the year. In the islands under the equator the heaviest fall is between October and February.

69. Such are the climatal conditions of the principal regions of Asia, under which the plants and animals that inhabit them are at present distributed. In attempting to appreciate and to explain the very complicated facts of distribution it is essential to bear in mind that what we find at the present time is, as was before observed, the result of causes that have been in operation from periods long antecedent to that in which the earth has taken its existing form, and acquired its existing conditions of temperature, climate, and arrangement of land and sea areas. Our knowledge of the manner in which the successive changes which have affected the earth s surface took place is, however, still so imperfect, that it is often not possible to state with certainty how the facts of distribution have occurred, and much is yet open to conjecture. But there is, notwithstanding, an overwhelming force of argument to establish the conclusion, that the diffusion of the forms of animal and vegetable life has gone on for a vast length of time by natural descent, and subject to the action of tendencies to variation ; the general result being that the forms which first existed have been suppressed, and others introduced in their places. This modification of form in time is seen to have been commonly accompanied by a corresponding movement or diffusion in place, governed no doubt primarily by the variations of temperature and climate and conditions of surface which have accompanied the movements of the solid crust of the earth, or may have been due to cyclical change. The conformity of the facts of the geographical distribution of life with this conception is no longer seriously questioned. The mutual relations among the several branches of animal and vegetable life, and the marked effects produced on all organised creatures by conditions of climate, are apparent. The abundance of certain forms of animals and plants in certain areas, and their gradual diminution in number beyond such areas until they disappear altogether, is well known; as also the ordinary similarity of the general assemblages of living creatures in countries not far distant from one 