t30 



NA TURE 



[December 12, 1901 



filled with mercury, one limb dipping into a vessel of 

 mercury, a^ and the other into a bath of dilute acid, /', 

 the level of the mercury being considerably higher than 

 that of the acid. The mercury is prevented from 

 syphoning over by drawing out the end,//, of the syphon 

 into a capillary tube. A contact is sealed into the top of 

 the syphon at / and a second contact is made to the 

 acid at j. When a diflference of potential is set up 

 between / and j in such a direction that / is positive to 

 ;', the capillary forces are overcome and the mercury 

 syphons over ; in so doing the mercury as it flows out of 

 h falls on to a delicately balanced lever, X-, which is thereby 

 tilted and makes contact with a stop, o, thus closing a 

 local relay circuit. The level of the mercury in a is 

 maintained constant by means of the arrangement shown 

 to the right of the syphon ; a reservoir, .f, is partly filled 

 with mercury, which is held up by keeping a partial 

 vacuum in /• ; when the level in a sinks the end, ;/, of the 

 side tube, /, is opened, thus allowing a certain amount of 

 air to enter and causing mercury to flow out until it again 

 closes the aperture. 



In a modification which has been devised the lever is 

 arranged outside and beneath the vessel /', which is then 

 provided with a capillary hole at the bottom just above 

 the end of the lever. The bottom of the vessel is in 

 this case covered with mercury, over which acid is poured 



KiG. 2, — " Armorl" Kleclro-capillary Relay. Balance t'orm. 



until the forces of gravity and capillarity are just balanced. 

 When more mercury syphons over a corresponding 

 amount escapes through the hole in the bottom of /' 

 and actuates the lever. The lever may here be replaced 

 by two platinum points, the falling mercury being then 

 made to bridge the gap between the points and thus 

 complete the relay circuit. 



An alternative form of the apparatus is shown in 

 Fig. 2. A glass tube, y, is balanced on a knife edge, -, 

 and is filled with mercury except for a drop of acid in 

 the centre at d. The current is led into this tube 

 through metal rods, 4, 4, dipping into mercury cups, 5, 5. 

 If a current is passed through the tube the meniscus 

 between the mercury and acid is displaced in the direc- 

 tion of the current and the balance is consequently dis- 

 turbed, as one arm now contains more mercury than the 

 other ; the pointer, 7, is deflected and makes contact 

 with either the stop 8 or 9 and thus closes the local circuit. 

 The construction is, however, said not to be so satisfac- 

 tory as that shown in Fig. i. It is claimed that the 

 apparatus is e.xtremely sensitive and very trustworthy in 

 its action, and even that it could be used as a substitute for 

 the coherer in atheric telegraphy or the syphon recorder 

 in cable work ; but these claims remain to be established 

 in practice. 



NO. 1676, VOL. 65] 



THE OASIS OF KHARGA} 



TOURING the last few years the Survey Department 

 ^—^ of the Public Works Ministry of Egypt has shown 

 considerable activity in the prosecution of investigations 

 connected with the geological survey of the valley of the 

 Nile, and the publications which it issues from time to 

 time show that tlie results which it obtains from them 

 are of great interest and importance. Until compara- 

 tively recently the conclusions formed about the strati- 

 fication of Egypt and its past geological history were based 

 upon researches which were undertaken without sufficient 

 preparation, often indeed without sufficient knowledge 

 on the part of those who made them, and the statements 

 made on the subject were often confusing and sometimes 

 contradictory. Under the direction of Sir W. Garstin, 

 however, things have taken a turn for the better, and the 

 geological publications prepared with his sanction and 

 approval really help to put our knowledge of the geology 

 of Egypt upon a sure base. 



The publication before us, by Mr. John Ball, is interest- 

 ing from every point of view and reflects great credit upon 

 the department to which he belongs. There is much in 

 it, of course, which will appeal only to the engineer and 

 geologist who are concerned with the practical administra- 

 tion of the district of the Oasis of which it treats, but 

 there is also much which will claim the careful attention 

 of the archaeologist and antiquary. The work is divided 

 into four chapters, which treat of the surveying methods 

 employed and their general results, of the roads between 

 the Nile \'alley and the Kharga Oasis, and of the topo- 

 graphy and geology of the Oasis ; besides these we 

 have an introductory chapter, five appendices, nineteen 

 maps and plates, and sixteen illustrations. The book is 

 satisfactory because it tells us, not only what are the 

 results which have been obtained, but also hou they have 

 been arrived at, and the plans, maps, and illustrations 

 enable the reader to follow these results with ease. 



The Oasis of Kharga has been a source of wonder 

 to untold generations of men, and the curiosity of all 

 cultivated students has been roused more and more 

 as each traveller has returned from it and unfolded 

 in his written descriptions of the place stories of its 

 people and antiquities. Concerning the origin of the 

 Oasis experts are in doubt, but Mr. Ball thinks that its 

 whole area has undergone disturbance which has resulted 

 in folding and faulting ; and since the faults aflfect the 

 highest rocks on the plateaux, it is clear that they took 

 place since the deposition of all the strata which are now 

 found in the Oasis, the calcareous tufa, of course, ex- 

 cepted. The date of the folding and faulting cannot be 

 fixed precisely ; all that can be said from the examina- 

 tion of the Oasis itself is that it took place since Lower 

 Eocene times. It is possible that it may be connected 

 with some younger faulting seen in the Nile \alley at the 

 First Cataract, but we have as yet insufficient informa- 

 tion for a definite connection of this faulting with the 

 folding of the strata in the Oases. The faulting produced 

 much cleaving and crushing of the rocks, but we have to 

 find out what hms the particular agency which excavated 

 and carried away the cracked-up limestones, and to 

 account for disintegration and removal of hundreds of 

 cubic miles of limestone rock, some of it being of con- 

 siderable hardness. It is probable that the excavation 

 was begun by the action of water, and that after this 

 ceised, owing to a total change in the clinjatic conditions, 

 /■('., the change from a moist climate to a dry one in 

 Egypt, the work was continued and is still going on by 

 the agency of wind and sand. The superficial erosion 

 both of the Oasis and of the hills within and round 

 about it is due to wind-borne sand, but this has never 

 been realised by travellers, for they have usually visited 



I " Kharga Oasis ; ils Topography and Geology." liy J. Ball. (Cairo, 

 1900.) Pp. 82. 



