739.]
[Footnote 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.]
[Footnote 40: ‘Manual of Ancient Geography’ (London, 1891), p. 311.]
[Footnote 41: _Geogr. Journ._, 56 (1920), p. 94.]
[Footnote 42: ‘Mysteries of the Libyan Desert’ (London, 1925), p. 303.]
[Footnote 43: _Geogr. Journ._, 43 (1924), p. 288.]
[Footnote 44: ‘The Secret of the Sahara—Kufara’ (London, 1921), p. 235.]
[Footnote 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.]
[Footnote 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.]
[Footnote 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.]
[Footnote 48: See Jordan’s notes on pp. 204 and 208 of his “Physische
Geographie und Meteorologie der libyschen Wüste,” Cassel, 1876.]
[Footnote 49: _Geogr. Journ._, 49 (1917), p. 55.]
[Footnote 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.]
[Footnote 51: _Geogr. Journ._, 47 (1916), p. 209.]
[Footnote 52: _Geogr. Journ._, 51 (1918), p. 252.]
[Footnote 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.]
[Footnote 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).]
[Footnote 55: Hurst, _op. cit._, p. 28.]
[Footnote 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?]
[Footnote 57: _Geogr. Journ._, 35 (1910), p. 385.]
[Footnote 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.]
[Footnote 59: _Zeitschrift der Gesellschaft für Erdkunde zu Berlin_,
1902, p. 760.]
[Footnote 60: ‘Recent Determinations of Geographical Positions in the
Libyan Desert,’ Survey Department Paper No. 34, 1919. p. 14.]
[Footnote 61: _Geogr. Journ._, 44 (1924), p. 377.]
[Footnote 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.]
[Footnote 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).]
[Footnote 64: ‘Recent Determinations of Positions in the Libyan Desert,’
Survey Department Paper No. 34 (Cairo, 1919), p. 12.]