ever
been removed.

Peering into the cavity thus disclosed I saw, to my surprise, what
appeared to be a small iron lever, thickly rusted, descending into some
cog-wheeled mechanism of a very complicated character.

"Now, watch the lake while I reveal to you its mystery," my companion
said, placing his hands upon the lever. With a harsh, grating noise it
fell back beneath the weight he threw upon it, and the harsh jarring of
cog-wheels revolving sounded for a few moments beneath our feet. Then, as
he set the mechanism in motion, my gaze was fixed upon the lake and I
stood aghast in wonderment.

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 nu