November 14, 18S4. 



SCIENCE 



457 



of other natural dams ; and these we find 

 repeated between Khartum and Berber, be- 

 low Abu-Hamed, between that and Dongola, 

 and between Dongola and Wadi-Halfa. At 

 the last-named place is found what is called 

 the second cataract ; and still farther down the 

 course of the river, at Assuan, is the well- 

 known ' first cataract.' Thence to the sea the 

 course of the great river is unobstructed in its 

 flow, except by the works of man. The great 

 viceroy, Mehemet-Ali, caused, at immense 

 cost, the construction of the famous barrage 

 clu Nil (' the dam of the Nile') a few miles 

 to the north of Cairo, in the endeavor to make 

 art complete, by a dam, what nature had so 

 well done in Central Africa and Nubia for 

 securing regular irrigating-supplies. 



The cataracts which play so important a 

 part in the preservation and regulation of the 

 Nile flow, are formed b} T masses of granite 

 rock, which at intervals cross the course of 

 the stream, making enduring dams. It is 

 easy to perceive, that, should the} T be worn 

 away or destroyed, the flow of the river would 

 be made much more rapid during the seasons 

 of high water ; and the Nile would become, in 

 Nubia, a fierce torrent during high water, and 

 a nearly dry channel for a considerable portion 

 of the year. 



The natural destruction of these great clams 

 by the formation of pot-holes, and the friction 

 of debris passing over them, is, from the nature 

 of the rock, very slow. From such observa- 

 tions as have been made, it is probable that 

 the natural wearing-away hardly exceeds six 

 feet in one thousand years ; and there is a 

 corresponding effect in the natural rising of 

 the river-bed below the cataracts and in the 

 delta by the deposit of silt from the turbid 

 waters. 



The Nile is navigable at all seasons of the 

 year, by steamboats of light draught, from the 

 mouth to Assuan (the first cataract) , between 

 the first and second cataracts (Assuan to 

 Wadi-Halfa) , between near Berber and Khar- 

 tum, between Khartum and a point a little to 

 the south of Gondokoro, and between Duffli 

 and Lake Albert. It is only during the season 

 of high water that boats can descend the Nile, 

 passing the cataracts between Berber and 

 Assuan. 



The great danger to boats descending these 

 fierce rapids during high water is found in the 

 eddies near the river-banks, islands, and large 

 rocks. The current is so rapid, and the fric- 

 tion on either hand so great, that the water 

 seems to heap up in mid-channel, where the 

 current is the strongest ; and great skill on the 



part of the steersman, and prompt and vigor- 

 ous work on the part of the engineer of the 

 steamer, or oarsmen of a row-boat, are neces- 

 sary to keep the boat on the ridge of the cur- 

 rent. If the boat is permitted to slide off this 

 ridge, she is quickly caught b} r the eddies, and 

 almost invariably lost. This is so well under- 

 stood by the Nubian boatmen, that, while 

 they work with a will at the oars in these 

 descents, they alwa} T s have their personal 

 effects packed in a snug parcel beside them, 

 ready to seize ; and they leap overboard, each 

 with his parcel on his head, the moment the 

 boat gets into a hopeless position. 



The work of towing or warping boats up 

 against the current is more difficult, but far 

 less dangerous, than the descent. 



Chas. P. Stone. 



A MUSSULMAN PROPAGANDA. 



The attention of geographers has of late 

 been particularly attracted by the operations 

 of a Mussulman confraternity known as the 

 Senousians, or the Brotherhood of Sidi Mo- 

 hammed Ben Ali es-Senousi, the founder of 

 the order. Of this now powerful and widely 

 ramifying society, Henri DuvejTier has recently 

 given an account. Its operations are of im- 

 portance to civilization, not merely from the 

 relation of this order to existing religions, but 

 from that which it bears to the efforts being- 

 made by civilized nations to develop the dark 

 continent, and explore its geographical and 

 other mysteries. The success of the religious 

 propaganda which the society represents men- 

 aces not only projected explorations, but the 

 very existence of established colonies and inter- 

 national traffic. It is believed that to their in- 

 stigation is due the melancholy fate of man} T 

 African explorers of late } T ears, among whom 

 may be mentioned Dournous Dupere, Beur- 

 mann, Von der Decken and his paily, Col. 

 Flatters, Capts. Masson and Diarnous, Dr. 

 Guiard, Beringer, Roche, Mademoiselle Tinne. 

 Sacconi, and others. If the present crusade in 

 the Sudan be not wholly due to their machina- 

 tions, it has at least been actively assisted and 

 impelled by individual members of the society, 

 and guided b} T the blind fanaticism which is its 

 rule of conduct. The favorite motto of the 

 head of the order declares Turks and Christians 

 to be equally offensive, and doomed to an equal 

 and simultaneous destruction. Their monas- 

 teries and influence extend from Morocco to 

 Arabia, and from the Mediterranean to Mozam- 

 bique, and govern two or three millions of peo- 



