“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.

CHAPTER XXIX.

A MUD BATH.

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.

“Now then, professor,” panted Zeb, “take hold on my feet and work along toward the edge of the hole with me.”

The professor seized Zeb’s boots with the grasp of a drowning man.  The branch cracked ominously.

“Easy thar, professor,” warned Zeb earnestly.  “Don’t pull more’n you can help or we’ll both be in the soup.”

The professor lightened his grip and slowly, hand over hand, Zeb began the slow journey back along the branch.  It was a feat only possible to a man whose muscles were of iron.  And before it was over even Zeb was almost overcome.  Perspiration streamed from his forehead and soaked his shirt as he dropped from the branch, having accomplished the journey and pulled the professor to the bank.

[Illustration:  The professor seized Zeb’s boots with the grasp of a drowning man.—­Page 240.]