[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.]