specimens in the other. He rushed up to the wall and stood for a
minute with his head on one side, like an inquisitive bird.

"Too bad. That stone's a large flat one and goes right through the
center of the wall," he mused. "The wall must come down."

And then, to the boys' consternation, he began demolishing the wall,
pulling down the stones and throwing them right and left.

"Professor, you'll get in trouble," warned Dick in alarm. "Those
cattle will get out. The farmer will be after us."

But the professor paid not the slightest attention. Taking off his
coat, he resumed his operations with even greater vigor than before.
The cattle in the field eyed him curiously. Then they began to move
toward him. In front of the rest of the herd was a big
black-and-white animal with sharp horns and big, thick neck.

It gave a sudden bellow and then rushed straight at the considerable
gap the man of science had made in the stone fence.

"It's a bull!" yelled Dick suddenly. "Run, professor! Run or he'll
toss you!"

With lowered horns the bull rushed down upon the unconscious scientist
at locomotive speed. But the professor was oblivious to everything
else but uncovering the odd-looking green stone embedded in the heart
of the wall.

The boys shouted to him but he didn't hear them. On rushed the bull,