In the morning Auda had us afoot before four, going uphill, till at last we climbed a ridge, and plunged over, down a sand slope. Into it our camels sank knee-deep, held upright despite themselves by its clinging. They were able to make forward only by casting themselves on and down its loose face, breaking their legs out of it by their bodies’ weight. At the bottom we found ourselves in the head-courses of a valley, which trended towards the railway. Another half-hour took us to the springing of this, and we breasted the low edge of the plateau which was the watershed between Hejaz and Sirhan. Ten yards more, and we were beyond the Red Sea slope of Arabia, fairly embarked upon the mystery of its central drainage.