Note 54 — 11. The Sand-Dunes.

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).