The equipotential surfaces, shown by dotted lines, will be squeezed together over the dune, and the electric forces, shown by full lines and arrows, being everywhere normal to those surfaces, will converge to the dune. Thus the dune will attract any particles carried by the wind and charged by conduction to the potential of the air conveying them. If the wind changes its direction, the attraction will still persist unaltered. Another point worth mention is that a sand-dune may be far more effective in its attraction of sand than a rocky ridge of the same size and shape would be; for when the electrified grains approach the surface of the dune, uncharged grains can rise from the dune to meet them; possibly some of the dancing of sand-grains which is frequently observable along the crest of a dune during a wind may be due to this cause.[56] During violent sandstorms, particles of sand may be whirled aloft and kept suspended at high altitudes for a period long enough for them to acquire very high potentials, and a sudden cessation of the storm may cause them to fall so rapidly that much of the charge may be retained; in which case, of course, electrical attraction may cause a very considerable deviation from the purely gravitational paths of the falling particles, the deviation being always towards the dune. Since the closeness of the equipotential surfaces and the mobility of the sand-grains composing the dune are greatest along the dune-crest, the maximum rate of deposition will be at the top of the dune, which is also the place where the wind exerts its greatest action in conveying the sand of the dunes southward. That the long axes of the lines of dunes do really correspond with the direction of the prevailing wind is certain, both from Mr. Beadnell’s observations of wind-direction[57] and from the fact that the scoring of the limestones in the Libyan Desert by wind-borne sand coincides in direction with the lines of dunes.[58] Thus I suggest that while the extension of the lines of dunes southwards is purely the result of the prevailing north-north-west wind, their clean-cut character and narrow width is the consequence of lateral attraction by the dunes themselves on the flying electrified particles of sand.
If the hypothesis I have ventured to put forward is regarded as likely to furnish the true explanation for the formation of the lines of dunes, I hope that someone possessing the necessary skill in electrostatic measurement will put it to a crucial test, by carrying out observations on and near the dunes themselves. If a number of flying sand-grains could be caught from different levels in an insulated metallic receiver connected to an electrometer, the readings of the electrometer would enable the potentials to which the grains were charged at different levels to be calculated, provided the electrostatic capacity of the receiving system and the number and diameter of the grains giving up their charge were measured. Allowance would, of course, have to be made for any electrification of the receiver by the air, or by uncaught sand-grains, as well as for losses by imperfect insulation of the collecting system. In a preliminary experiment of this kind which I made in Cairo during a sandstorm, leakage from the electrometer was found to be so great as to vitiate any attempts at accurate measurement; but I think this particular difficulty could be overcome with a specially designed apparatus, or that at least the leakage could be measured and allowed for. The taking of observations of any kind in dune areas during windy weather is a most difficult and trying operation, and commonly means, in addition, a sojourn of some weeks in isolated and otherwise uninteresting areas. But the advent of the motor-car has so greatly facilitated communication in the deserts that difficulties of access are now not so great as they used to be; the south end of the 35-kilometre line of the Kattania Dunes, for instance, about 90 kilometres west-south-west of Cairo, which would form an excellent site for detailed observations, can now be reached by a four or five hours’ car journey from Cairo instead of the several days’ journey by camel which was formerly necessary.
12. The Distribution of Stone Implements.
I have not made any special study of stone implements, nor have I been able to pay any considerable attention to their collection during my journeys, having usually found my time very fully taken up with other matters. I have, however, been much struck by the wide distribution of stone implements in the desert. I have found them, for instance, not only in Siwa Oasis and near the wells of Abu Mungar and Sheb, but also on the plateau between Baharia and Farafra, on the open desert between Terfawi and Owenat, and to the south-west of Owenat near the Anglo-French boundary. This wide range of occurrence, coupled with the finds of Schweinfurth and others on the plateaux nearer the Nile, inclines me to think that there is scarcely any part of the Libyan Desert in which stone implements might not be found by an expedition which would make the collection and study of them one of its main objects. The likeliest places in which to search (besides the neighbourhood of old wells and springs) would appear to be the shores of the various lakes and around the feet of hills affording shelter from wind and sun; for it is in such localities that I have most usually come across implements and pottery. Grinding-stones, often with a sort of stone rolling-pin, unpolished celts, knives, and arrow-heads are the principal forms of implement I have met with. Many of the grinding-stones must have been carried for a considerable distance, for they are made out of rocks which do not naturally occur in the localities in which they are found.
Whether this wide distribution of stone implements would justify the view of Blanckhenhorn[59] that primitive man lived on the desert plateaux rather than in the Nile Valley must, I think, be settled by the further collection of specimens and by a careful comparison of the forms to be found in the two situations. In this connection I may remark that the grinding-stones I have seen in the desert seemed to me to be very similar to those used by the Nubians of the Nile Valley to-day for grinding corn.
APPENDIX
SOME RECENTLY DETERMINED POSITIONS IN THE LIBYAN DESERT
In view of their possible utility in connection with future exploratory surveys, I give below a list of some of the more important of the positions which I have recently determined by astronomical observation in the southern part of the Libyan Desert, with brief descriptive notes on the places to which they refer. The observations were made whilst accompanying H.R.H. Prince Kemal el Din Hussein on his exploratory motor-car expeditions of 1923, 1924, and 1925. The longitudes of Qasr Farafra, Abu Mungar, Pottery Hill, and Regenfeld depend on the transport of a box-chronometer; the others on wireless time-signals from Europe. The positions of peaks were found by triangulation from the actual observation-spots. The altitudes of camps are from careful barometric determinations; those of the peaks depend on trigonometric levelling from the camps.
| Place. | Lat. N. | Long. E. | Altitude above sea. Metres. | |||||
|---|---|---|---|---|---|---|---|---|
| ° | ′ | ″ | ° | ′ | ″ | |||
| Merga, Camp 600 metres S.W. of west corner of salt lake. (Lake surface is 17 metres below level of camp.) | 19 | 2 | 29 | 26 | 18 | 32 | 526 | |
| Bir Terfawi, Camp close to well and palms | 22 | 55 | 12 | 28 | 52 | 51 | 244 | |
| Gebel Kissu, Summit | 21 | 34 | 59 | 25 | 8 | 26 | 1726 | |
| Gebel Owenat, highest point | 21 | 54 | 34 | 25 | 0 | 47 | 1907 | |
| „ „ remarkable triple peak | 21 | 53 | 51 | 25 | 1 | 39 | 1718 | |
| „ „ S.W. peak | 21 | 49 | 35 | 24 | 53 | 52 | 1450 | |
| Chunk Hill, Summit | 21 | 52 | 46 | 25 | 13 | 56 | 985 | |
| Owenat, Camp 600 metres S.E. of western spring, on plain at foot of mountain | 21 | 48 | 35 | 24 | 51 | 45 | 568 | |
| Owenat, Camp at mouth of gully, about 800 metres S.S.E. of eastern springs | 21 | 53 | 8 | 25 | 7 | 58 | 626 | |
| Gebel Kamil, Summit (160 metres above plain) | 22 | 16 | 31 | 26 | 38 | 11 | 800 | |
| Regenfeld, Rohlfs’ cairn of 1874, rediscovered 1924 | 25 | 10 | 49 | 27 | 24 | 22 | 470 | |
| Pottery Hill, Summit (39 metres above plain) | 24 | 26 | 27 | 27 | 38 | 54 | 506 | |
| Sheb, Camp close to well, 240 metres N.N.E. of fort | 22 | 19 | 48 | 29 | 45 | 46 | 228 | |
| Mut, Government Rest-house | 25 | 28 | 37 | 28 | 58 | 24 | 119 | |
| Abu Mungar, Camp at small ruin, 400 metres E.S.E. of well | 26 | 30 | 22 | 27 | 35 | 29 | 117 | |
| Qasr Farafra, Camp E. of village, 200 metres S.W. of Tomb of Sheikh Dakhil | 27 | 3 | 26 | 27 | 57 | 52 | 90 | |
Merga.—To geographers the most important of the new determinations will undoubtedly be that of the uninhabited oasis of Merga, which had not previously been visited by a European, and of which the situation could hitherto only be guessed at from Arab statements. Merga lies in a shallow depression about 50 metres deep, broken by sandstone hills and sand-dunes, and extending for some 20 kilometres north-east and south-west. The salt-lake, near the centre of the depression, measures about 300 metres by 150. It is surrounded by tall rushes and sandhills except at its south-western end. There are numerous date-palms, both near the lake and at considerable distances from it, as well as acacia trees and tamarisk bushes. The neighbourhood of the lake swarms with mosquitoes. Good and plentiful water was got by shallow digging about 1 kilometre south-south-east of the observation-spot, and could probably be obtained by digging almost anywhere in the depression. It is possible that the names Bidi and Tura el Bedai, shown with a question mark on some Sudan maps, may refer to different spots within the same depression. Owing to the plentifully scattered vegetation, the place cannot easily be missed, either by travellers passing within several miles of the lake, or by aircraft; but the landing of aeroplanes in the neighbourhood of the lake might be difficult owing to the extensive sand-drifts.
Bir Terfawi.—Scarcely less important than the accurate fixation of Merga is that of Bir Terfawi, the farthest south-west of all the Egyptian artesian water-sources hitherto known. It will be remarked that this latest determination places the well some 22′ south and 15′ east of the position which I had provisionally assigned to it from the rough data furnished by Lieut. Moore’s traverse of 1916.[60] A knowledge of the true position of Terfawi is specially desirable for a traveller who wishes to reach it, owing to the absence of any conspicuous landmark near it and to the sandy nature of the surrounding country, which causes tracks to be soon obliterated. There are numerous sandhills covered with tamarisk bushes around Terfawi, and a little grazing for camels. Besides the well at which observations were taken, water was found by digging in the sand at the foot of some tamarisk mounds about 13 kilometres farther west, and it is probable that good and plentiful supplies could be obtained at shallow depths near any of the other mounds. The palmtrees at Terfawi are few and small, and are less conspicuous than the tamarisk-bushes. These latter should enable the place to be easily found by aircraft; but landing would require some caution owing to the prevalence of drifted sand.
Gebels Kissu and Owenat.—The peaks of these mountains (especially that of Kissu, because of its isolated character and sharply marked summit) will form useful points for the connection of future surveys, being visible from very long distances.
Chunk Hill is a prominent isolated hill, nearly conical and of dark colour, which forms a good landmark in the broken country to the east of Gebel Owenat. It rises some 335 metres above the ground at its foot.
The Springs of Owenat.—Of the two water-sources of Owenat whose positions are given, the western one is the better, and is moreover very easy of access, being practically on the level of the plain which extends southwards from the mountain mass, and easily discoverable by the numerous animal-tracks converging to it; it is a pool among great boulders, obviously fed by percolation through cracks and fissures in the granite mountain which towers above it. The eastern water-source is less easy of access; it lies about 1 kilometre up a stony gully, and consists of a series of pools in the rocky floor of the gully, fed by trickling springs at the level where the granite is overlain by sandstone. The plain to the south of the western spring would form an excellent landing-ground for aircraft.
Whilst in the neighbourhood of Owenat, I had hoped to re-determine the longitude of Hassanein Bey’s camp of 1923 with the aid of wireless time-signals, or at least by triangulation-connection to one of my observation-points, because a really accurate fixation of the longitude of a point about midway along his route from Jarabub to Furawia would possibly furnish the means of slightly correcting the longitudes assigned to Hassanein’s camps at Kufra and Erdi. I was unfortunately prevented from carrying out the desired connection; but from a hurried car-traverse which I made, skirting the western side of the mountain mass, in the course of which I must have passed pretty close to the site of Hassanein’s Owenat camp, I think that camp really lies about in longitude 24° 49′, or some 5 miles to the west of where I had previously calculated it to be from Hassanein’s traverse data;[61] and that in consequence the longitudes assigned to Kufra and Erdi from the same data may be some 2 or 3 miles too great.
Gebel Kamil is a sharply pointed hill of dark sandstone, forming a useful landmark between Terfawi and Owenat. It was visible from the east for more than 40, or from the west for about 20, kilometres. The name was given to it by Prince Kemal el Din in honour of his father, Hussein Kamil, the late Sultan of Egypt.
Regenfeld, it will be remarked, was found to be very nearly in the position assigned to it by Jordan in 1876,[62] and my estimation of the level of the place is only 20 metres higher than Jordan’s. The neighbouring dune I found to be 30 metres high, agreeing exactly with Jordan’s measurement of fifty years previously; and as far as I could judge, the situation of the dune relative to the cairn seems to have remained unchanged through this long interval, showing that there has been at any rate no great lateral displacement of the dunes. The iron tanks left by Rohlfs were quite intact, in spite of their half-century of exposure; they had become covered with a hard dark brown film, apparently of magnetic oxide of iron, not rusted in the ordinary way, a fact which speaks strongly for the dryness of the region. Empty wine-bottles left by the Rohlfs expedition were frosted by the sand-blast wherever they were exposed; but surprisingly little of the glass had been removed in this way.
Pottery Hill is the northern one of a pair of nearly conical dark sandstone hills about 40 metres high, which are conspicuous from some distance owing to their situation on a nearly level sand-plain. I gave it its name from the numerous jars which I discovered at its foot in 1917.
Sheb Well is a pool of fairly good water in a shallow excavation at the foot of a clump of dom palms, half surrounded by sand-dunes and tamarisk mounds, 240 metres north-north-west of a fort; the latter occupies a fairly conspicuous position on a sandstone hill about 25 metres high. Another well, situated at the foot of a clump of dom palms about 11 kilometres farther north, is called Bir Terfau; this should not, of course, be confused with the Bir Terfawi already mentioned.
To the north of Sheb, on the Arbain road, are three other water-sources which I visited in 1925 and of which I determined the approximate positions by a carefully controlled car-traverse: Bir Kassaba, in lat. 22° 41′, long. 29° 55′; Bir Abu Hussein, in lat. 22° 53′, long. 29° 55′; and Bir Murr, in lat. 23° 22′, long. 30° 5′. Bir Kassaba is a pool of good water at the foot of a clump of palms; Bir Abu Hussein, which likewise yields good water, is a small excavation in sand near a hill of pink coarse-grained granite, and usually requires to be dug out afresh by each traveller; Bir Murr, on a little plain surrounded by hills, consists of several holes, with fairly good water, excavated in the sand at the south side of the outcrop of a steeply inclined bed of speckled calcareous sandstone.
I have not myself traversed the desert to the east of Sheb, but according to an intelligent Arab who has recently made the journey from Sheb to Dungul, there are only two spots between these places at which water is obtainable; they are Bir Abu Seifa, a small well of good water about 50 kilometres from Sheb, and Bir Haleifa, about 15 kilometres farther on, where there is a fort and numerous wells. This latter place must, I think, be the same as the Bir Nakhlai, of which the position found astronomically by Colonel Talbot in 1893 was lat. 22° 29′ 1″, long. 30° 19′ 36″.
The Rest-house at Mut, in Dakhla Oasis, is a low whitewashed building of three rooms, situated on open ground a little to the south of the village. It is conspicuous by reason of its isolation from other buildings, and forms a convenient starting-point for car journeys to the south-west of Dakhla, as well as for journeys to Kharga by the southern track which was mostly followed by cars during the war.
Abu Mungar proved to be very close to the position given by Mr. Harding King’s observations of 1912.[63] To the south-east of Abu Mungar, about halfway to Dakhla, are two remarkable hills forming outliers of the plateau. These hills, which are good landmarks, were first discovered by Mr. Harding King, and as they appear to possess no native name, I have called them “King’s Hills” on the map. The farther one of the two from the plateau lies in lat. 25° 58′, long. 28° 11′, from compass-bearings which I took to it on a careful car-traverse in 1924. The other is about 2 kilometres farther north.
Qasr Farafra.—The position of the village of Qasr Farafra which I gave in my paper of 1919[64] is proved to be very nearly correct by the latest observations. The main advantage of the new determination is that it is referred to an easily identifiable landmark, the tomb of Sheikh Dakhil, instead of to the ill-defined centre of the village.
Levels in or near Baharia Oasis.—I think it may be well to record here that owing to an unfortunate mistake about the datum to which the level of the old military railway-terminus at B6 was referred, all the altitudes given on pp. 10 and 11 of my above-mentioned Survey Department Paper of 1919 require a correction of 19 metres to be added. At the time when I triangulated Baharia in 1917, I was informed that the level found by the military authorities for the terminus was measured from the sea-level datum, and I employed the value given for it as my fundamental level. But I have since ascertained that an arbitrary datum was employed by the military engineers; and trigonometric levelling from the Nile Valley has shown that this arbitrary datum was about 19 metres above sea-level.
FOOTNOTES:
[1]The use of motor cars for reconnaissances in the Libyan Desert, initiated by the British Army during the Great War, has been considerably developed in the last few years by Prince Kemal el Din Hussein (son of the late Sultan of Egypt), who has made several long exploratory journeys with a fleet of cars of the caterpillar type, specially constructed so as to be capable of carrying considerable loads over sandy tracts which were impassable to the Army patrols. The expeditions organized and led by His Royal Highness have resulted in important additions to geographical knowledge concerning the Libyan Desert, especially the southern and western regions, which have hitherto been practically inaccessible. I had the pleasure of accompanying the Prince on three of his journeys, and am much indebted to him for thus affording me the opportunity of making many observations which would otherwise have been impossible to me. I have further to thank him for kind permission to illustrate this paper by reproductions from a selection of the excellent photographs taken by him during our travels.
[2]Kindly communicated by Mr. A. R. Boyce, Director of Sudan Surveys.
[3]Printed by the French Army Staff, 1926.
[4]By way of comparison, it may be remarked that the area (at the sea-level contour) of the Qattara depression is about four times that of the Dead Sea and Jordan Valley depression; but the latter (reckoning to the lowest point of the bed of the Dead Sea) is about six times the deeper of the two.
[5]A similar ambiguity of meaning very commonly attaches to Arabic geographical terms. Thus beled may mean either a village or an entire country; gebel, either a mountain or a desert; gezira, either an island or a part of the Nile bank which is alternately covered and uncovered by the rise and fall of the river; bir, which properly signifies a well, is often used to denote a spring or a water-cistern; wadi, though its proper meaning is a valley, is also sometimes employed for a closed-in depression, as in the case of the Wadi Natrun.
[6]‘Nouveaux Mémoires des Missions de la Compagnie de Jesus dans le Levant,’ Tome ii. (Paris, 1717), p. 74. This little book contains a map of Egypt, prepared by Father Sicard at Cairo in 1715, on which the “Mer sans eau” is shown with “bateaux petrifiez” depicted alongside it.
[7]See “Particulars concerning the Valley of the Natron Lakes, and that of the Old Bed of the River” (with map) by Andreossi, General of Artillery, in ‘Memoirs Relative to Egypt, Written in That Country during the Campaigns of General Bonaparte in the Years 1798 and 1799, by the Learned and Scientific Men who Accompanied the French Expedition’ (London, 1800), p. 270.
[8]See Rohlfs’ ‘Drei Monate in der libyschen Wüste’ (Cassel, 1875), p. 337; and Jordan’s ‘Physische Geographie und Meteorologie der libyschen Wüste’ (Cassel, 1876), p. 214.
[9]‘Zeitschrift der Gesellschaft für Erdkunde zu Berlin,’ 1902, Tafel III.
[10]‘Topography and Geology of the Fayum Province of Egypt’ (Cairo, 1905), p. 67.
[11]Since the above was written, the Professor of Geography at the new University of Cairo, M. Lorin, has suggested that at a former geological epoch the Nile flowed through Siwa Oasis and thence westward so as to make Cyrenaica an island. (See ‘L’Egypte,’ published by the Institut Français d’Archéologie Orientale, Cairo, in December 1926, p. 153.) M. Lorin does not give any grounds for this suggestion, and I know of no evidence that would justify its acceptance. As will be gathered from my remarks elsewhere in this paper, it seems to me far more likely that Siwa and the other northern depressions were in recent geological times regions of central drainage, than that they were local deepenings of a single channel; and in regard to earlier geological periods we possess too little information to form any conclusions as to drainage-lines.
[12]See Sir Hanbury Brown’s ‘The Fayum and Lake Moeris’ (London, 1892), pp. 40-48 and 106-110; Sir W. Willcocks’ ‘Perennial Irrigation and Flood Protection for Egypt’ (Cairo, 1894), also the same author’s ‘Egyptian Irrigation,’ third edition (London, 1913); Sir W. E. Garstin’s ‘Report on the Basin of the Upper Nile’ (Cairo, 1904), Appendix I. pp. 6-9; and Beadnell’s ‘Topography and Geology of the Fayum Province of Egypt’ (Cairo, 1905), pp. 16-24.
[13]‘Report on the Administration of the Irrigation Department for 1895’ (Cairo, 1896), pp. 25-28.
[14]‘Report on the Administration of the Irrigation Department for 1896’ (Cairo, 1897), p. 129.
[15]It may be remarked that even if a channel were to be cut from Lake Mariut to the Wadi Natrun it would still be necessary to continue pumping on a large scale from Lake Mariut to maintain the present-day drainage. According to returns communicated to me by the Irrigation Service, the quantity of water removed from Lake Mariut by pumping during the last nine years has averaged nearly 650 million cubic metres a year; and even on the most optimistic estimate of evaporation and seepage the Wadi Natrun could not be expected to dispose of half this quantity after the lapse of the few years which would be occupied in filling it to a permanent level.
[16]The average discharge of the Nile past Cairo in a year is 67 cubic kilometres. The capacity of the Qattara depression at the sea-level contour is roughly about 900 cubic kilometres.
[17]Geogr. Journ., 56 (1920), pp. 97-99 and 161-163.
[18]Lucas (‘Natural Soda Deposits in Egypt’ (Cairo, 1912), p. 15) regarded the water of the Wadi Natrun as being due to infiltration from the Nile, mainly on the ground that he supposed the water visibly entering the wadi (as small streams and trickles) to do so on the north-eastern side. But this, I think, is a mistake, as it neglects consideration of the large springs of warm fresh water which bubble up in and near the salt-lakes themselves, and also the large wells of the monasteries which lie south-west of the lakes.
[19]A far better site for this well would have been in the depression some 20 kilometres to the north-west, the ground-level at the lowest point of which is only 61 metres above sea; but at the time when the site of the well was chosen the existence of this depression was unknown.
[20]See Appendix III. in the second volume of Willcocks’ ‘Egyptian Irrigation,’ 1913, p. 853.
[21]Schweinfurth (op. cit., p. 855) even thought it might have percolated to Siwa, in spite of the great distance (480 kilometres) and the adverse direction of the difference of level between the two places.
[22]Mr. Beadnell (‘An Egyptian Oasis’ (London, 1909), p. 139) estimates the total discharge of the Kharga wells at 53,000 cubic metres per diem. We should not, I think, err greatly in estimating the total output of all the Egyptian oasis wells and springs at about five times this, or 250,000 cubic metres per diem. As the average daily discharge of the Nile in Egypt is about 180 million tons, a local influx into it several times as great as the total discharge of the oasis wells would represent but an insignificant addition to the discharge of the Nile. I think, however, that the influx of the warm water might possibly be detected by careful thermometric measurements made in the river near its banks at the time of low Nile.
[23]This was the lowest level recorded at the time when I drew the static contours on the map. The further exploration of the depression by Mr. Walpole in the present year has shown that at one place the ground-level descends to − 134 metres; the situation of this local deepening is, however, such that the drawing of the static contours would not have been affected by its inclusion in the list of adopted data.
[24]‘An Egyptian Oasis’ (London, 1909), p. 156.
[25]“The Mutual Interference of Artesian Wells,” Geol. Mag. (London, 1909), pp. 23-26; also ‘An Egyptian Oasis,’ chap. x.
[26]‘An Egyptian Oasis,’ p. 140.
[27]‘Drei Monate in der libyschen Wüste’ (Cassel, 1875), p. 115.
[28]See Jordan’s ‘Physische Geographie und Meteorologie der libyschen Wüste’ (Cassel, 1875), p. 204. Jordan states that there were on an average not more than two to four plants per acre in this locality.
[29]See Rohlfs’ ‘Drei Monate in der libyschen Wüste’ (chapter ix. of which was written by Ascherson), p. 250.
[30]In my paper on ‘Recent Determinations of Geographical Positions in the Libyan Desert,’ published in Cairo in 1919, I gave (p. 14) the approximate position of the hill as lat. 24° 28′, long. 27° 45′. The position given above is from a more accurate determination which I made on revisiting the place with Prince Kemal el Din in 1923. On this latter occasion the excavations made by the Prince in the sand around the foot of the hill revealed the existence of hundreds of additional jars, many of them intact, set in regular order in the sand and obviously forming a water-dump. The majority of the jars bore clearly incised markings, which Prince Kemal el Din was able later to identify positively as being tribal marks of the Tebus, the marks being in fact exactly the same as those used by the Tebu tribes of the present day.
[31]See Jordan’s ‘Physische Geographie und Meteorologie der libyschen Wüste’ (Cassel, 1876), p. 215.
[32]‘Kharga Oasis’ (Cairo, 1900), p. 57.
[33]Geogr. Journ., 42 (1913), p. 283; also ‘Mysteries of the Libyan Desert’ (London, 1925), p. 304.
[34]This “locus” is of course given by joining up the points of intersection of the 100-metre water-contour with the 200-metre ground-contour, the 200-metre water-contour with the 300-metre ground-contour, and so on.
[35]It is possible that the “Nubian Sandstone” in South-Western Egypt may represent more than one geological system, as has been found to be the case in Sinai. Col. Tilho (Geogr. Journ., 56 (1920), p. 259) records that the sandstone at Ennedi proved to be of Upper Silurian age by the fossils found in it.
[36]Though we passed within 10 kilometres to the east of Kissu on the return journey from Merga to Owenat in 1925, I could not observe the structure of the mountain very clearly, owing to haze. But it appeared to me to be a huge granitic intrusion rising through the gneiss which covers a large area hereabouts. The sandstone over which we had been travelling since leaving Merga gave place to gneiss about 40 kilometres before we came abreast of Kissu, and gneiss continued to be the country rock until we came within 30 kilometres of Gebel Owenat, when sandstone reappeared, at first capping low hills and then forming the plain.
[37]There exists no published English translation of Ptolemy’s ‘Geographia.’ Müller’s edition (Paris, 1883-1901) gives Greek and Latin texts, with a Latin commentary.
[38]Quoted in Müller’s edition of Ptolemy’s Geography, p. 739.
[39]For the latest delineation of this region, see the hypsometric map in the ‘Atlas de l’Algérie et de Tunisie,’ published by the Cartographic Service of the Algerian Government, Paris, 1924, Fascicule II.
[40]‘Manual of Ancient Geography’ (London, 1891), p. 311.
[41]Geogr. Journ., 56 (1920), p. 94.
[42]‘Mysteries of the Libyan Desert’ (London, 1925), p. 303.
[43]Geogr. Journ., 43 (1924), p. 288.
[44]‘The Secret of the Sahara—Kufara’ (London, 1921), p. 235.
[45]Beadnell, “The Sand-dunes of the Libyan Desert,” Geogr. Journ. 25 (1910), pp. 379-395; Harding King, “The Nature and Formation of Sand Ripples and Dunes,” Geogr. Journ., 47 (1916), pp. 189-209; Harding King, “Study of a Dune Belt,” Geogr. Journ., 51 (1918), pp. 16-33, and Discussion, pp. 250-258.
[46]For particulars of the distribution and directions of the dune-lines near Gebel Owenat, and between that mountain and Sarra, I am indebted to Prince Kemal el Din Hussein, who devoted special attention to observing them on his expedition to Sarra in 1926.
[47]It may be remarked that in the desert of Northern Sinai the dune-lines follow curved directions, instead of maintaining a single direction as in the Egyptian Libyan Desert, and this curvature of the Sinai dune-lines has generally been thought to be caused by the deflecting influence of the mountain masses of Moghara, Yelleg, and Hellal on the wind; but here also there may be regional influences at work.
[48]See Jordan’s notes on pp. 204 and 208 of his “Physische Geographie und Meteorologie der libyschen Wüste,” Cassel, 1876.
[49]Geogr. Journ., 49 (1917), p. 55.
[50]The steep Miocene escarpment, some 200 metres high, which bounds the Qattara depression on the north, probably produces a great eddy in the wind sweeping southwards over it, causing a whirl round a horizontal axis south of the scarp; this may be very influential both in excavation and in lifting the sand to great heights. As a large portion of the floor of the depression at the foot of the escarpment is formed of a salty sludge (sabakha), it is likely that a considerable quantity of finely divided salt crystals are carried up with the sand in summer, and this may conceivably act to some extent as a binding material in the dunes.
[51]Geogr. Journ., 47 (1916), p. 209.
[52]Geogr. Journ., 51 (1918), p. 252.
[53]Many years ago I discovered that this film can be removed by treating the sand with warm hydrochloric acid, leaving the grains of snowy whiteness (see my ‘Description of the First or Aswan Cataract’ (Cairo, 1907), p. 57). But I think traces of iron in the ferrous state must exist even within the white siliceous body of the grains, and be continually passing into the ferric state at the exposed surface; for otherwise it is difficult to account for the persistence of the yellow colour in spite of the mutual rubbing of the grains during their transport by wind. The film may possibly have an influence on the electrical behaviour of the grains, by reason of its possessing a higher conductivity than the silica of the interior.
[54]Professor P. E. Shaw has quite recently (Nature, vol. 118 (1926), p. 659) suggested from laboratory experiments that the mutual impact of particles of identical nature in sandstorms may result in electrification of the particles and of the air with charges of opposite signs. I think most travellers in the desert have, like myself, formed the impression that electrification of the air is more pronounced during sandstorms (khamsins) than at other times. But it is curious that observations made with the electrograph at Helwan Observatory do not seem altogether to bear this out. Dr. Hurst finds that khamsins are usually accompanied by low values of potential gradient in their early stages, with great disturbances both positive and negative for some hours before and after the break of the khamsin (‘Observations on Atmospheric Electricity at Helwan Observatory,’ Survey Dept. Paper No. 10 (Cairo, 1909), pp. 52-62).
[55]Hurst, op. cit., p. 28.
[56]Mr. Harding King records in his paper of 1915 that he found sand-grains blown off the dunes to be electrified; can these have been grains which became charged by jumping up from the dune to meet others and thus sharing their charge?
[57]Geogr. Journ., 35 (1910), p. 385.
[58]The hot and disagreeable south and south-west winds which blow during the “khamsins” of the spring are much less constant in their direction; and though they produce thick sandstorms, they have always seemed to me to occasion rather a general scattering of the sand over the surface of the desert than to form deposits having very definite arrangement; moreover, they blow on relatively few days of the year. The far more prevalent cool strong north-north-west winds, on the other hand, often scarcely vary a point in direction for days together, and their coincidence with the direction of the dune-lines is very pronounced.
[59]Zeitschrift der Gesellschaft für Erdkunde zu Berlin, 1902, p. 760.
[60]‘Recent Determinations of Geographical Positions in the Libyan Desert,’ Survey Department Paper No. 34, 1919. p. 14.
[61]Geogr. Journ., 44 (1924), p. 377.
[62]Jordan’s final position was lat. 25° 11′ 7″, long. 27° 25′ 0″. The longitude given on the document found in the bottle at the cairn (14° 42′ E. of Berlin, or 28° 3′ 42″ E. of Greenwich) was a preliminary value resulting from Jordan’s observation of a lunar distance, and was wrong by some 40 miles.
[63]Mr. Harding King’s observations gave the latitude as 26° 30′ 46″, and the longitude as 27° 36′ (see his map in Geogr. Journ., 42 (1913), p. 516).
[64]‘Recent Determinations of Positions in the Libyan Desert,’ Survey Department Paper No. 34 (Cairo, 1919), p. 12.