Perdiccas crossed one of the branches of the Nile, and then led his army forward to attack Ptolemy in a strong fortress, where he had intrenched himself with his troops. The forces of Perdiccas, though much more numerous than those of Ptolemy, fought with very little spirit; while those of Ptolemy exerted themselves to the utmost, under the influence of the strong attachment which they felt for their commander. Perdiccas was beaten in the engagement; and he was so much weakened by the defeat, that he determined to retreat back across the river. When the army arrived at the bank of the stream, the troops began to pass over; but after about half the army had crossed, they found, to their surprise, that the water, which had been growing gradually deeper all the time, became impassable. The cause of this deepening of the stream was at first a great mystery, since the surface of the water, as was evident by marks along the shore, remained all the time at the same level. It was at length ascertained that the cause of this extraordinary phenomenon was, that the sands in the bottom of the river were trampled up by the feet of the men and horses in crossing, so that the current of the water could wash them away; and such was the immense number of footsteps made by the successive bodies of troops, that, by the time the transportation had been half accomplished, the water had become too deep to be forded. Perdiccas was thus, as it were, caught in a trap--half his army being on one side of the river, and himself, with the remainder, on the other. He was seriously alarmed at the dangerous situation in which he thus found himself placed, and immediately resorted to a variety of expedients to remedy the unexpected difficulty. All his efforts were, however, vain. Finally, as it seemed imperiously necessary to effect a junction between the two divisions of his army, he ordered those who had gone over to make an attempt, at all hazards, to return. They did so; but in the attempt, vast numbers of men got beyond their depth, and were swept down by the current and drowned. Multitudes of the bodies, both of the dead and of the dying, were seized and devoured by the crocodiles which lined the shores of the river below. There were about two thousand men thus lost in the attempt to recross the stream.