At Wiseman's name mentioned in such a connection both the boys started.
"What! they have gone?" thundered the Arab chief.
"Yes, sir," stammered Diego, his coward nature aroused at the sight of his superior's fury.
"And by this time they are rifling the ivory cache. That fool Wiseman shall pay dearly for this. Bring him to me," shouted the Arab.
Desperate as was the boys' position they could not restrain a start of amazement as Professor Wiseman, his face pale as ashes to his very lips, came tremblingly forward.
"You were attached to this boys' camp to prevent by all means their sailing till I attacked the camp and made them prisoners, were you not?" demanded Muley-Hassan angrily.
Wiseman stammered something in reply.
"You are a coward as well as a fool," went on the slave-dealer, a cruel sneer breaking over his face; "but you have blundered for the last time. Take this fool away and kill him!" he ordered, turning away as if there was an end of the business.
Pitiful cries broke from the lips of the unhappy professor as he heard his death-warrant thus pronounced. He threw himself on his knees and begged and pleaded in a loud screeching tone for a little more time. But the chief was obdurate.
"Take him away," was all he said, and his men, not daring to disobey his orders any longer, fairly dragged the unfortunate prisoner toward the river bank. There was a short, sharp scream that chilled every drop of blood in the boys' bodies and then a splash. Professor Wiseman had paid the price of his treachery.