The other catastrophe afflicted the Khalifa, and its tale was brought to the advancing army by the Intelligence spies, who to the last—­even when the forces were closing—­tried to pass between them.  Not content with building batteries along the banks, Abdullah, fearing the gunboats, had resolved to mine the river.  An old officer of the old Egyptian army, long a prisoner in Omdurman, was brought from his chains and ordered to construct mines.  Two iron boilers were filled with gunpowder, and it was arranged that these should be sunk in the Nile at convenient spots.  Buried in the powder of each was a loaded pistol with a string attached to the trigger.  On pulling the string the pistol, and consequently the mine, would be exploded.  So the Khalifa argued; nor was he wrong.  It was resolved to lay one mine first.  On the 17th of August the Dervish steamer Ismailia moved out into the middle of the Nile, carrying one of the boilers fully charged and equipped with pistol detonator.  Arrived at the selected spot, the great cylinder of powder was dropped over the side.  Its efficiency as a destructive engine was immediately demonstrated, for, on the string being pulled by accident, the pistol discharged itself, the powder exploded, and the Ismailia and all on board were blown to pieces.

Undeterred by the loss of life, and encouraged by the manifest power of the contrivance, the Khalifa immediately ordered the second of the two boilers to be sunk in the stream.  As the old Egyptian officer had been killed by the explosion, the Emir in charge of the arsenal was entrusted with the perilous business.  He rose, however, to the occasion, and, having first taken the precaution of letting the water into the boiler so as to damp the powder, he succeeded in laying the second mine in mid-stream, to the joy and delight of Abdullah, who, not understanding that it was now useless, overwhelmed him with praise and presents.

Beguiled with such stories and diversions, the day of rest at Wady el Abid passed swiftly.  Night brought beetles, bugs, and ants, and several men were stung by scorpions—­a most painful though not dangerous affair.  Towards morning it began to rain, and everyone was drenched and chilled when the sun rose across the river from behind a great conical hill and dispersed the clouds into wisps of creamy flame.  Then we mounted and set out.  This day the army moved prepared for immediate action, and all the cavalry were thrown out ten miles in front in a great screen which reached from the gunboats on the river to the Camel Corps far out in the desert.

When we had advanced a little further, there arose above the scrub the dark outlines of a rocky peak, the hill of Merreh.  The whole of the 21st Lancers now concentrated, and, trotting quickly forward, occupied this position, whence a considerable tract of country was visible.  We were hardly twenty-five miles from Khartoum, and of that distance at least ten miles were displayed.  Yet there were no enemy.