51 = 



NATURE 



[Sept. 21, 1882 



Common, it would be premature to decide upon present data. 

 The Brazilian Telegraph Company has cabled from Madeira two 

 positions for September 12 and 13, obtained on II. M.S. 

 Triumph by Capt. Markham, but taken in conjunction with 

 the Rio message, they tend to throw a doubt upon the 

 accuracy of some of the figures so far received. 



Another comet discovered by Mr. Barnard is announced in a 

 telegram from Boston, U.S., to Dunecht, it is described as 

 circular, 2' in diameter, with some central condensation. An 

 observation at Harvard College gives the following position : — 

 Sept. 14-8162 G.M.T. : R.A. 7I1. 19m. 17*, Decl. + i6°3'5l". 

 Daily motion in R.A. : + Im. 44s. ; in Decl. : + 43'. 



There is no possibility of identity of this comet with that of 

 1812. 



The Total Solar Eclipses of iS83 and 1885. — Observers 

 of the total solar eclipse on May 6, 1883, will have but very 

 limited observing-room, and in fact will be confined for stations 

 to one or two of the smaller islands or inlets of the Marquesan 

 group, the Roberts Islands of the Admiralty Chart. Wide 

 separation of the parties to secure better chances of favourable 

 weather will therefore be impracticable. The same contingency 

 will occur in the case of the total eclipse next following, vi:. 

 that of September 8, 1SS5, where the course of the central zone 

 is again almost wholly a sea-track. In this case observers will 

 be limited to the southern parts of the north island of New 

 Zealand, and to the extreme northern point of the southern 

 island. 



The following figures will indicate the precise conditions : — 



Latitule, Latitude, Latitude, 



Long. E. N. limit Central S. limit 



of totality. Eclipse. of totality. 



171 ... - 39 31-4 ... - 40 20'2 ... - 41 I02 

 173 ... 39 42-0 ... 40 31-0 ... 41 2PO 



;i7S ••• 39 55'° •■• 4° 43'S ... 41 34 - 4 

 177 ... - 40 9-5 ... - 40 5S-S ... - 41 49-2 



The duration of totality on the central line in longitude 175" 

 will be Im. 52s. 



GEOGRAPHICAL NOTES 



At the Southampton meeting of the British Association Mr. 

 Joseph Thomson read a paper On the Geographical Evolu- 

 tion of the Tanganyika Basin. The keynote of this paper is 

 struck by a reference to a recent lecture of Dr. Archibald Geikie. 

 to the Royal Geographical Society, in which he points out that 

 the days are now over in which the scientific geographer is 

 content with the simple description of the superficial aspects of 

 the various regions of the globe. He must also know how they 

 came to be, and what they have been in the past. This line of 

 inquiry is applied by Mr. Thomson to the lake regions of Central 

 Africa, but more particularly to the Tanganyika Basin. In the 

 6rst place he presented a bird's-eye view of the lake regions 

 from the Indian to the Atlantic Ocean, bringing into relief only 

 the most prominent features of the geography, but describing 

 more in detail the aspect of the Tanganyika Basin, round which 

 the chief interest centres. From a description of these purely 

 superficial matters he proceeded to describe what these have 

 been in the remote past, and the manner in which they have 

 been evolved, being of course compelled to call in the assistance 

 of the sister science geology. The conclusions he arrived at as 

 to the primary origin of the region are, from purely hypothetical 

 considerations, based on the theory of a shrinking nucleus, and 

 the necessary effects on the earth's crust arising therefrom. At 

 a later stage, however, he is on safer grounds when he is able 

 to appeal to the rocks themselves as to the aboriginal con- 

 ditions of the African continent south of the Equator. 

 These, according to Dr. Thomson, prove the existence of 

 an immense central sea cut off from the ocean by the eleva- 

 tion of the continent, and which was almost coterminous 

 with the present drainage area of the Congo. An elevated 

 ridge was upheaved along the eastern boundary of this sea, the 

 origin of the trough of Tanganyika, by the collapse of the centre 

 of this ridge and the central sea, subsequently drained away to 

 the west, leaving Tanganyika isolated. Mr. Thomson then pro- 

 ceeded to describe how its secondary characters arose, and its 

 scenery was moulded, by the action of sub-aerial denudation on 

 rocks of different powers of resisting the decomposing and 

 eroding agents, and explained the curious marine-like type of its 

 shells, the origin of its outlet, the Lukuga, the freshening of the 



water of the lake, and finally the curious intermittency of the 

 outflow. The various stages in the evolution of the Tanganyika 

 Basin were summarised as follows : — The first appearance of the 

 future continent, we have been kd to believe from various 

 theoretical considerations, was the appearance of a fold of the 

 earth's crust bounded by two lines of weakness converging 

 towards the south, which fold gradually rose till it appeared 

 above the ocean, first along these two lines of weakness, in the 

 form of a series of islands, which finally join, inclosing in their 

 centre a large part of the ocean. This inclosed water area 

 formed a great central sea, and the inclosing land along the lines 

 of weakness is now indicated by the east and west coast ranges. 

 In the second stage the continent of Africa south of 5 N. lat. 

 presented the outline of the continent of to-day. The third 

 stage shows the central plateau with the great central sea very 

 uracil diminished in size, and almost coinciding with the present 

 Congo Basin. There is as yet no evidence of the existence of 

 Tanganyika. After an enormous period of undisturbed deposi- 

 tion of sand in the sea, the fourth stage is ushered in by a period 

 of great continental covulsions. On the line of the future 

 Tanganyika a huge boss of rock is intruded into the throbbing 

 crust, and the surrounding region elevated to a considerable 

 extent, followed by the subsequent collapse of the body of the 

 elevated area originating the great abyss of Tanganyika. The 

 fifth great stage is marked by the formation of a channel through 

 the western coast mountain, causing the draining of the great 

 central sea, which immediately becomes the inner drainage area 

 of the Congo. The sixth stage then sees Tanganyika isolated as 

 a lake by itself, from which time dates the moulding of its 

 present scenery, the formation of an outlet, the freshening of 

 its waters, and the lowering of its level, and finally we have 

 seen that the intermittency of the lake's outflow is explained by 

 the probable fact that the rainfall and evaporation nearly balance 

 each other in ordinary seasons. 



The Geneva correspondent of the Times sends some notes on 

 an interesting paper recently read by Prof. Calladon : — "So far 

 back as 1S80 M. de Saussure suggested the probability of the 

 level of Lake Leman being much lower than it had been a few cen- 

 turies previously, and that there had been a time when the upper 

 part of Geneva formed a peninsula, washed on every side except 

 that of Champel, by the waters of the lake. This theory has 

 lately been comfirmed by the observations of Prof. Calladon, 

 who, at the recent meeting of the Association of Swiss Geo- 

 graphical Societies, read a paper on the subject that attracted 

 much attention. It results from the Professor's investigations 

 that the Plateau of the Tranchee, to the south-east of the city, 

 and the hill once crowned by the temple of Diana, and now by 

 the cathedral, are parts of the same lacustrine terrace, both being 

 composed of regular beds of sand and gravel, having an inclina- 

 tion of 30 to 37 degrees, and dipping in a north-westerly direc- 

 tion. Superimposed on these beds is a horizontal layer of 

 pebbles of an average thickness, much exceeding the thickness of 

 the oblique layers underneath. The height of this layer, Prof. 

 Calladon contends, corresponds with the former maximum level 

 of the lake, which was 28 to 30 metres higher than the present 

 level. The excavations for the foundation of the new theatre 

 which were laid in 'drift,' had to be carried to a great depth, 

 and included 3000 square metres of ground. It was quite 

 evident from the nature of the deposits, which had not been pre- 

 viously disturbed, that hereabouts the Arve, once upon a 

 time, joined the Rhone, and other excavations have indicated 

 the old course of the former river to the point at which it 

 now takes its departure. Underneath the inclined bed of 

 pebbles and gravel comes glacial clay, identical in every 

 respect with the glacial clay that now underlays the bed ot 

 the Rhone. It is from the depth of drift resting upon this plat- 

 form of gravel clay that Prof. Calladon calculates his estimate 

 of the lowering of the level of Lake Leman in modern times. 

 He is confirmed in his conclusion by the fact that the deposits in 

 the ancient bed of the Arve are not alone similar in kind to the 

 deposits still brought down by the river, but identical with them 

 in chemical composition. In the opinion of Prof. Calladon, 

 Geneva, at a period not many centuries before the Christian era, 

 occupied a strategic position analogous to that of many other 

 cities of antiquity in being built upon a promontory almost sur- 

 rounded by « ater. The uncovering of the platform of glacial 

 clay enables Prof. Calladon further to ascertain the minimum 

 level of the lake at the time when the superincumbent layers of 

 Alpine sand and gravel were brought down by the Arve. It 

 follows, from the geometric measurements which have been 



