I refused to seem curious, and walked beside him in silence.

“He studies sleeping sickness.  If he can find the key to the solution of that scourge it will mean promotion for him.  He has noticed that the sleeping sickness is always at its worst beside the lake, and putting two and two together like a sensible man has reached the conclusion that the disease may be propagated in some way in the blood of these things.”

We emerged into a clearing in which a pool more than a hundred yards long and nearly as many wide was formed naturally by a hollow in the surface of a great sheet of granite.  The pool was fed by a trickle of water from a jumble of rocks at one end.  At the other end the bottom of the pond sloped upward gradually, so that a ramp of smooth rock was formed, emerging out of shallow water.  A stone wall had been built about three feet high to enclose that end of the pond, and all the way along both sides the granite had been broken and chipped until the edges were sheer and unclimbable.

“Look!” he said, pointing.

I looked and grew sick.  On the ramp, half in the water and half out lay about a hundred crocodiles basking in the sun, their yellow eyes all open.  They were aware of us, for they began to move slowly higher out of water as if they expected something.

“You see that post?” asked Schillingschen.

The stump of a dead tree that he referred to stood up nearly straight out of a crack in the rock, and a few yards above water level.  The crocodiles all lay nose toward it, some of them twelve or fourteen feet long, some smaller, and some very small indeed, all interested to distraction in the dead tree-trunk.

“That is where he feeds them,” Schillingschen announced.  “He has tested them for hearing, smell, and eyesight.  By making fast a living animal to that post be has been able to convince himself that from about nine in the morning until five in the afternoon their senses are limited.  Only occasionally do they come and take the bait between those hours.  They are hungriest in the early morning just before daylight.  Recently a large ape tied to the post at midday was not killed and eaten until four next morning, and that is about the usual thing, although not the rule.  Now my proposal is—­”

He stepped back and eyed me with the coldest look of appraisal I ever sickened under.  I blenched at last—­visibly suffered under his eye, and he liked it.

“—­that you tell your secret or be fastened to that post from noon, say, until the crocodiles make an end of you!”

He stepped back a pace farther, perhaps to gloat over my discomfort, perhaps from fear of some concealed weapon.

“You have not much time to arrive at your decision!”

He took another pace backward.  It occurred to me then that he was looking for some one he expected.  Nobody turning up, he began to gather loose stones and throw them at the reptiles, driving them down into deep water, first in ones and twos and then by dozens.  Most of them swam away to the far side of the pool, and hid themselves where it was deep.