528 



NATURE 



\Oct. 12, 1876 



although not yet capable of demonstration by the recorded 

 observations of the rain-gaus^e. 



There are two regions especially where the facts already 

 collected show most clearly not only a diminution in the 

 amount of water existing on the surface, but a most 

 striking one. In Central Asia and in Western North 

 America, the observations of numerous observers all point 

 unmistakably in this direction. The observations of the 

 Schlagintweits in Thibet and Turkistan are referred 

 to. In all portions of High Asia, south and north of the 

 main water-shed, in Thibet, throughout the entire longi- 

 tudinal depression between the chain of the Himalaya 

 and the main water-shed of the Karakorum, there are 

 numerous places where the former existence of mountain 

 lakes may be recognised. In Western Thibet the evapo- 

 ration exceeds the supply of water, so that the prevailing 

 condition is at the present time one of gradual diminution 

 in the area covered by water. There seems to be here, 

 in combining alllhe results of the Messrs. Schlagintweits' 

 observations, abundant evidence of a marked change of 

 climate in the most recent geological period — resulting in 

 the almost entire disappearance of extensive lakes — and 

 also that this desiccation is still going on. 



The observations of Mr. Drewin his elaborate work on the 

 Jummooand Kashmirterritories,fully corroborates theoften 

 previously expressed opinion, that the Valley of Kashmir 

 was, in later geological times, completely occupied by a lake. 

 But no evidence has, as yet, been discovered to prove that 

 this desiccation took place during the historical period, 

 although the traditions of the natives point in that direc- 

 tion. There is, however, abundant proof of diminution 

 in the area covered by water in the basin of the Aral and 

 Caspian Seas, not only during the latest geological epoch, 

 but also within a comparatively recent period. Those 

 who wish to investigate the matter will find the material 

 in a paper by Major Wood, published in the Journal of 

 the Royal Geographical Society for 1875, and we may 

 state in his recent work on " The Shores of Lake Aral," 

 and in the articles contributed by him to Nature. There 

 is no doubt of the former vastly greater extension of the 

 Caspian and Aral Seas ; it seems beyond dispute that a 

 gradual desiccation of the region has been in progress, and 

 that it is still going on. That there once existed here a vast 

 Asiatic Mediterranean which was connected by navigable 

 waters with the Northern Ocean is very generally admitted. 



Similar facts in regard to the diminished quantity of 

 water in Arabia are cited by various travellers in that 

 country. In Africa the existence of extensive ruins in 

 the Great Libyan Desert, in a region quite destitute of 

 water, and which is now entirely uninhabited, may be 

 taken as a strong indication of great changes since the 

 historic period. Dr. Livingstone, in his travels in South- 

 ern Central Africa, was again and again much impressed 

 with the proofs presented to him of a rapid and extensive 

 diminution within recent times of the amount of water in 

 the lakes and rivers of that region. 



Prof. Whitney adduces much evidence to show that a 

 similar state of things exists in America, especially in the 

 region west of the Rocky Mountains, and above all in the 

 " Great Basin." For example, the terraces surrounding 

 Great Salt Lake are so conspicuous, that no traveller 

 passing through that region on the railroad could fail to 

 notice them. It is certain that the sharp and well-defined 

 character of the terraces in some parts of the western 

 region indicates very clearly that the diminution of the 

 volume of the water must have been an extremely recent 

 phenomenon. It is doubtful whether this desiccation has 

 any connection with the former glaciation of the regions 

 in question; so far as the problem under discussion is 

 concerned, it is of no consequence. 



It is certain that both in Asia and North America the 

 phenomena of desiccation are on too grand a scale by far 

 to be supposed to have anything to do with cutting down 

 of forests. The drying up has been commenced before 



man interfered with nature, and has been continued with- 

 out reference to his puny operations. 



Evidence is adduced to prove that within the historical 

 period, the volume of several of the European rivers 

 has considerably decreased. In this connection the in- 

 vestigations of Berghaus on the Rhine, the Elbe, and the 

 Oder are referred to. Berghaus shows that each of these 

 rivers had decreased in volume during the past hundred 

 years, and that there was reason to fear that they would 

 eventually have to disappear from the list of the navigable 

 streams of Germany. Gustav Wex came to the same 

 conclusion with regard to the Danube. 



The general impression, both of Mr. Wex and a com- 

 mittee of the Vienna Academy, seems to be that the 

 cutting down of the forests is the essential cause of the 

 desiccation. But the number of facts which can be given 

 in support of this hypothesis is quite small. That a 

 positive diminution in the average quantity of water carried 

 down in the streams would necessarily ensue on removing 

 a portion of the forests in any region, Prof. Whitney does 

 not consider to have been proved as yet. 



In regard to one question, this commission of the 

 Vienna Academy is quite unanimous, and this is that 

 great pains should be taken by the different Governments 

 of the enlightened States throughout the world to obtain 

 more light and additional data bearing on this subject. 

 If desirable for Europe, Prof. Whitney thinks it is still 

 more so in America. They need much more numerous 

 and more accurate observations of rainfall. If it can be 

 shown that the removal of the forests seriously diminishes 

 the quantity of water running in the streams, then there 

 is yet time to stay the hand of the wood-cutter ere the 

 mischief be consummated. 



That there has been a very marked decrease in the 

 amount of water on the earth within the most recent geo- 

 logical period is beyond a doubt ; and that there is con- 

 siderable reason to believe that the desiccation is still 

 going on has, we think with Prof. Whitney, been made 

 evident by the facts he adduces. He promises on 

 another occasion to discuss the connection of the so- 

 called " glacial epoch " with the present one of desicca- 

 tion. 



The subject is one of great interest and of prime im- 

 portance, both from a scientific and an economical point 

 of view. The New York Nation, in referring to Prof. 

 Whitney's paper, tries to account for the phenomenon as 

 follows : — 



Setting speculative causes aside, such as the possible 

 variation in the central heat beneath the earth's crust, 

 there is one well-known cause which, we think, can 

 scarcely be demonstrated to be incapable of producing 

 the desiccation. The sun's heat is notoriously the source 

 of all climates, and changes in the amount of heat radiated 

 from the sun are now regarded as causing the changes in 

 terrestrial weather. It is therefore reasonable to ascribe 

 our drying-up, since it requires ages for its completion, to 

 a change in the solar cause requiring also a long cycle for 

 its fulfilment, provided that astronomy gives us proof of 

 any such change. And astronomy does tell us of two 

 such cycles : one in the obliquity of the ecliptic, and one 

 in the perihelion distance of the earth from the sun, both 

 cycles being results of planetary perturbations of the 

 earth's orbit. The effect of the second of these cycles is 

 too abstruse to explain here ; the first is simpler. As the 

 angle between the plane of the earth's equator and that of 

 her orbit diminishes, the limits of the torrid zone also 

 diminish, inasmuch as that zone is bounded by the tropics 

 which are determined by the angle in question. The 

 region, then, over which the sun is occasionally vertical is 

 being narrowed. An obvious result of this narrowing 

 would seem to be an intensification of the equatorial 

 phenomena of trade- winds, heat, and rainfall within the 

 torrid zone, and a corresponding loss of heat and of pre? 

 cip^tation in the exira-tropical zones, 



