Solitary barchans, being well-defined isolated masses, lend themselves to observation better than the large and continuous belts of sand. They occur of all sizes, from little baby crescents, a metre or two across, to enormous masses 30 or 40 metres high and 200 to 300 metres in breadth. In all cases, except when their steady march southwards has been temporarily interrupted by southerly winds, the southern face, flanked by the horns of the crescent, is a straight slope of from 30 to 33 degrees, its inclination is, in fact, the angle of rest of loose dry sand, and it is formed of the sand which is continually blown up the opposite slope and dropped over the crest. In a moderate breeze the whole barchan progresses steadily in a southerly direction without loss or gain of sand; in stronger winds, while still continuing its course, the dune may at the same time be either losing or gaining in actual bulk. The rate at which the barchans travel varies with their size. The larger the dune the greater the amount of material to be moved, so that, as a general rule, small barchans progress much more rapidly than large ones. The actual rate may vary from 10 to 20 metres per annum, the average being about 17. For instance, the big isolated dune on the road between Headquarters and Kharga village moved 11 metres southwards between December 8, 1907, and December 8, 1908, while during the same period a small dune in the Bellaida district travelled 21 metres in the same direction. The dimensions of these two dunes is as follows: +------------------------------------+-------------+----------+ | | Kharga Road | Bellaida | | | Dune. | Dune. | +------------------------------------+-------------+----------+ | | Metres. | Metres. | | Vertical height of centre of crest | 17 | 10·5 | | Maximum breadth (east and west) | 259 | 160 | | Maximum length (north and south) | 230 | 70 | +------------------------------------+-------------+----------+ These are merely instances from dunes which I have had under observation for a number of months, full details of which I hope shortly to publish.