Hitherto the atmosphere had always been perfectly clear, but now it was thick and cold, the horizon wearing that gray, heavy aspect which in Europe precedes a fall of snow. No one, however, ventured to pronounce this word; it appeared an occurrence so unlikely in the plain, at such a season and under such a latitude. What, then, was our surprise, on awaking on the morning of the 19th of April, to find the tents covered with a thick sheet of snow, and to see the vast expanse of the desert white to the verge of the horizon, like the frozen steppes of Siberia! The general ordered the camp to be raised immediately, for the bivouac afforded very scanty materials for fire, and he hoped there might be wood in the mountains if he could reach them. The snow continued to fall in large flakes; the troops, anxious and sorrowful, described a thousand circuits and made a thousand useless turnings, for our Arab guides were utterly at fault. During three or four months previous to the expedition, Cavaignac had been selecting and retaining as guides whatever Saharians he could find acquainted with that part of the desert he intended to traverse. The Arabs are gifted with remarkable dexterity in steering without compass, recognising a footstep imperceptible to the common eye, scenting the water at a distance, and finding their way by marks which would escape the most observant European. A Saharian once affirmed to Colonel Daumas: 'I am not considered remarkably sharp-sighted, but I can distinguish a goat from a sheep at the distance of a day's journey; and I know some who smell the smoke of a pipe, or of broiled meat, at thirty miles! We all know each other by the track of our feet in the sand, for no one tribe walks like another, nor does a wife leave the same footprint as an unmarried woman. If a hare has passed, we know by its footprint whether it is male or female, and, in the latter case, whether it is with young. If we see the stone of a date, we know the particular tree that produced it.' Our conductors, though not pretending to all this sagacity, were nevertheless far in advance of some of us who proudly called ourselves 'old Africans,' and considered ourselves wonderfully expert in tracking the desert paths. But now the landmarks on which they depended had disappeared beneath the snow; and the atmosphere was so surcharged with it, that the mountain summits could no longer be descried. At length the guides abandoned the hopeless effort, and declared that they had entirely lost the way, and knew not in what direction to proceed. At this juncture, Cavaignac, remembering that the mountains had appeared due south on the preceding evening, seized his compass, and boldly ordered the troops in that direction. It was the only hope; but the march became so fatiguing, and the natives gave so little encouragement to the expectation of finding the mountains wooded, that a halt was ordered, and a bivouac on the snowy plain.