It is rather curious to note that although the sand-dunes form the greatest obstacle to motor-cars in the desert, yet it is chiefly by means of motor-cars that the true extent and distribution of the sand- dunes has been ascertained. Dunes are the most difficult of all desert features to map properly by ordinary reconnaissance methods with camel transport. Their smooth outlines provide no points on which intersections can be made, and no survey marks put on them will remain in place for more than a few hours, or at most a few days; they occur mostly in nearly level country, where it is impossible to find a station whence they can be overlooked; the absence of shadows on them renders it impossible to say whether one is looking at a single line of dunes, or at several lines, miles apart, one behind the other in echelon. The only sure way of mapping dunes is to traverse their sides along their entire length, and this is impracticable with camels owing to the enormous distances which would have to be covered without water. But with motor- cars one can run alongside them at 40 kilometres an hour instead of the camel’s 4, and their distribution can thus be rapidly and easily ascertained. [Illustration: _Prince Kemal el Din’s expedition of 1925 leaving Bir Terfawi for the waterless journey of 250 miles to Owenat_] [Illustration: _The well at Bir Terfawi in the palm clump_] [Illustration: _Jebel Owenat from the south: the triple peak (5635 feet) in centre_] [Illustration: _The lake at Merga located by Prince Kemal el Din’s expedition of 1925_]