As the lever was drawn and the rusty cogs ran into one another, the whole mass of rock damming the lake above the small cascade where it fell into the river, gradually rose, like a great sluice gate, allowing the waters to escape and empty themselves, roaring and tumbling, into the winding river beside which we had journeyed. It was an amazing transformation, as imposing as it was unexpected. A few seconds before, the river, shallow and peaceful, fed by its tiny cascade, rippled away over its pebbly bed; now, however, with the great volume of water from the lake it rose so rapidly that the swirling, boiling current overflowed its banks, sweeping everything before it. Nor was this the only result of pressing the lever, for at the opposite end of the lake a similar outlet opened, and as I looked I saw the water falling with a rapidity that was astounding. Hydraulic power was evidently known to these strange semi-civilized people, yet the actual means by which the lake was so rapidly emptied I was unable to discover, all the machinery being hidden away in some subterranean chamber. "By what cunning device is this accomplished?" I inquired of Omar, who stood regarding the disappearing flood with satisfaction. "This mechanism was invented ages ago by one of my ancestors," he answered. "Its exact date no man can tell. But here water is given mastery over itself, and so careful was its constructor to preserve the secret of its existence that the slaves and workmen, all criminals, were kept close prisoners during the whole time they were at work, and on its completion they were all, without a single exception, killed, in order that none should know the secret save the reigning Naba and his heir." "They were murdered then!" "They were all criminals who for various serious crimes had been condemned to death. It is said they numbered over two hundred," Omar answered. But even as he had been speaking the water of the lake had so drained away that its clean stony bottom was now revealed, the pebbles being exposed in large patches here and there, while the deeper pools remaining were alive with water-snakes and fish of all kinds. There seemed but little mud, yet in the very centre of the great basin was a patch of pebbles and rock higher than the remainder, standing like a small island that, before the lever had been touched, had been submerged. Leading the way, Omar descended to the edge of the lake, skirted it for some little distance, until he came to a long row of flat stones placed together, forming stepping-stones to the miniature island. "Come," he said. "Follow me," and starting off we were soon crossing the bed of the lake, being compelled to advance cautiously owing to the slippery nature of the weeds and water-plants that overgrew the stones. On gaining the island, however, a fresh surprise awaited me, for Omar, halting amid the mud in the centre, exclaimed: