But everything was ready and the boys were waiting impatiently half an hour later, and there was no sign of the professor.
Suddenly they heard his voice shouting to them from the distance.
The professor's shouts came plainly to their ears the next minute, borne on a puff of wind that swept through the canyon.
"Help! Help!" was the burden of his cries. "Get me out!"
"Now, what's happened to him?" demanded Zeb, with a trace of impatience.
"I don't know, but he must be in trouble of some sort," cried Jack.
"Maybe it's another donkey," mischievously suggested Dick.
The cries were redoubled. They waited no longer but started off across the island on the run. Zeb carried his big forty-four revolver.
The ground was rough and rocky but they made good time. Bursting through a screen of trees from beyond which came the professor's piteous cries, they received a shock.
The man of science was in the center of a large, round hole full of black mud that bubbled and boiled and steamed as if it were alive. All that was visible of the professor was the upper part of his body.
Seriously alarmed, the boys shouted to him to keep up his courage, and that they would get him out.
"How did you get in?" asked Zeb, cupping his hands.
"I fell in," rejoined the poor professor. "The ground gave way under my feet. Hurry and get me out, it's terribly hot."
They looked about them desperately for some means of extricating him from his predicament. But just at the moment none was offered, and with every struggle the professor was sinking deeper in the black, evil-smelling pool of mud.
"Gracious, what are we to do?" cried Jack in despair.
"He's too far out to reach him," said Zeb, equally at a loss.
"But we must do something," chimed in Tom.
Suddenly Zeb had an inspiration. A tree grew on the banks of the mud volcano, the sudden caving in of which, under the professor's weight, had precipitated him into it.
"If I could get out on that branch," said Zeb, "I might be able to bend it enough to bring my feet over him and then work back toward the edge of the mudhole."
"It's worth trying--anything is worthy trying," agreed Jack.
Zeb took off his coat and then shinned up the tree. Then, hanging by his hands he began working out along the branch. As he went it bent till it hung right over the mudhole. Before long his feet dangled above the professor's head.