Then it was committed to the desert. One hundred and eighty miles of trackless, parched waste lay between them and El Obeid. The first few days had indeed been weary work; the ground was full of broad, deep cracks, for it had been under water when the Nile rose, and on the river receding the fierce sun had had this effect upon the mud. Mimosa shrub also grew thickly in parts; and it was important that the men should not straggle, for that was the opportunity the Arabs were on the look-out for, and so many fearful disasters had already occurred from this very cause. For the soldiers, if the fierce children of the desert rushed upon them unexpectedly when they were in loose formation, were as helpless as sheep, though, when in a compact body, and under the immediate eyes of their English officers, they could fight steadily enough, as was proved at the battle of Marabia in the spring of that same year, when they inflicted very severe losses upon the Arabs, whom they totally defeated at little cost to themselves. But though the march had been toilsome, the river was near at hand, and the worst enemy of the desert, _thirst_, was not to be dreaded. But now they were to leave the Nile behind them, and depend for their water supply entirely on the wells, which were understood to be at certain places on the line of march, though these were often found to be at much greater distances than had been represented. The progress was very slow, for they had to march in square--the leading battalion in line, the rear also in line, the right and left faces moving in fours, or in column, according to circumstances. In the centre were the camels and other baggage animals, with the two things which were as necessary to existence as air to breathe--ammunition and water. When, through inequality of ground or any other cause, the lines bulged, or the columns were broken, it was necessary to halt till all closed up again, and this of course delayed the march very much. Ten miles a day were the utmost they could accomplish without running most unjustifiable risk. The irregular cavalry now proved of extreme value; preceding the army, scattered out in front and on each flank, they were bound to come upon any ambushed enemy in time to gallop back and warn the main body, who would then be able to close up, and present a front on every side, which the enemy would find no opening to break in at. On the fourth day, as the troops were passing over a plain of sand which stretched away to the horizon all round, without a shrub to break the monotony, only here and there a block of rock, or the skeleton of a camel, showing where some wretched overtried animal had sunk under the too great presumption upon his wonderful powers of endurance, the scouts gave notice of Arab approach, and a figure could be seen coming over the summit of a sand-hill, thus proving that the ground, though apparently flat, was undulating.