Part 7
It is noticeable, however, that to the north of the “great sand sea” of Rohlfs, which extends for an unknown distance westward, the known depressions are much shallower; and yet the total quantity of sand in this region is probably greater than that of all the more easterly dunes put together. The country between Jarabub and Jalo is however almost unexplored, though it is known to be very sandy and to be the theatre of violent sandstorms. Whether similar deep depressions to that of Qattara have been excavated along, or to the north of, the Jarabub-Jalo road, or whether the area of loosely compacted arenaceous rocks exposed there is so enormous as to have yielded such a quantity of sand without any very deep depressions being formed, is a point that can only be cleared up by further exploration in Cyrenaica.
We now come to the most puzzling of all the questions connected with the sand-dunes, namely, why do they persist as sharply defined long narrow lines in certain places, while the ground elsewhere is almost absolutely free from sand? As Mr. Harding King puts it, the dunes appear to have a curious power of collecting all the sand in the neighbourhood.[51] The sheltering effect of ripples in the dunes, and a “shepherding” effect of winds a point or two off the normal, have been suggested as possibly affording some explanation of this apparent collecting-power,[52] but neither of these suggestions appears to be adequate. I think a more direct explanation is possible; namely, that the dunes really _have_ the power of attracting sand from the air, and that this power is due to the well-known law of attraction between a conducting surface at zero potential and an electrically charged body. That air-borne sand-grains in the neighbourhood of a dune may carry electric charges was shown by Mr. Harding King’s experiments, recorded in his paper of 1916, and I have since found sand-grains to carry electric charges of high voltage during sandstorms in Cairo. As to how the grains become charged with electricity, it can hardly be due to friction against each other, for the sand is remarkably uniform in composition, consisting generally of rounded grains of almost pure silica coated with a thin translucent film of ferric oxide,[53] and for the production of frictional electricity it is generally regarded as necessary that the two bodies which are rubbed together should be of dissimilar materials;[54] nor is it likely that they are electrified by friction with the air, for they travel with it. But if the particles are carried into an upper air-stratum, they will gradually acquire its potential by conduction; and the ordinary potential gradient in the air in Egypt is over 100 volts per metre of height.[55] Hence a sand-particle carried along for some time in the air at a height of only 20 metres above the ground may become charged to 7 electro-static units of potential, or 2100 volts; and on approaching a dune rising to about that height it will be attracted to the dune, which being in connection with the earth is at zero potential. The attraction will be very small unless the sand-grain approaches very closely to the surface of the dune; from some calculations I have made, a spherical grain of silica half a millimetre in diameter charged to a potential of 2100 volts will not be attracted with a force equal to half its own weight until it approaches within a centimetre of the conducting surface, and the grain will have to approach within 7 millimetres of the surface before the attraction is equal to the weight of the grain. But the important thing to note is that the attraction is independent both of the sign of the charge and of the direction of the wind which carries the particle. Another way of looking at the matter is to consider the probable distribution of the equipotential surfaces in the air about a dune, as in the diagram below, which represents a section taken perpendicular to the direction of the wind and to the axis of the line of dunes:
[Illustration: _Diagram showing suggested distribution of equipotential surfaces and directions of lines of force in the neighbourhood of a sand-dune_]
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.
_Altitude _Place._ _Lat. N._ _Long. E._ above sea. Metres._
° ′ ″ ° ′ ″
Merga, Camp 600 metres S.W. of west 19 2 29 26 18 32 526 corner of salt lake. (Lake surface is 17 metres below level of camp.)
Bir Terfawi, Camp close to well and 22 55 12 28 52 51 244 palms
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 21 48 35 24 51 45 568 western spring, on plain at foot of mountain
Owenat, Camp at mouth of gully, 21 53 8 25 7 58 626 about 800 metres S.S.E. of eastern springs
Gebel Kamil, Summit (160 metres 22 16 31 26 38 11 800 above plain)
Regenfeld, Rohlfs’ cairn of 1874, 25 10 49 27 24 22 470 rediscovered 1924
Pottery Hill, Summit (39 metres 24 26 27 27 38 54 506 above plain)
Sheb, Camp close to well, 240 22 19 48 29 45 46 228 metres N.N.E. of fort
Mut, Government Rest-house 25 28 37 28 58 24 119
Abu Mungar, Camp at small ruin, 400 26 30 22 27 35 29 117 metres E.S.E. of well
Qasr Farafra, Camp E. of village, 27 3 26 27 57 52 90 200 metres S.W. of Tomb of Sheikh Dakhil
_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.
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