Then came a sharp crack. There was a sound as though a hundred pop-
guns were being fired, and the boat slackened speed.

"Look!" cried Harry pointing ahead "Our sail has burst, Bert"

"No, it's the sheet rope--the main rope that holds the sail fast-
that's broken," replied Bert. "Lucky it did, too, or we might have
gone over. I was going to let go of it."

The ice-boat slid along a short distance, and then came to a stop. The
sail, no longer held in place so as to catch the wind, was blowing and
flapping, making snapping sounds like a line of clothes in a heavy
wind.

"All right, girls, no danger now," called Bert, as he got out to make
the flapping sail fast again. As he looked at the end of the broken
rope he uttered a cry of surprise.

"Look here!" he called to Harry, "this rope has been cut!"

"Cut?"

"Yes. Someone hacked it partly through with a knife, and the wind did
the rest."

There was no doubt of it. The main rope had been partly severed, and
the strain of the hard blow had done the rest.

"That fellow we saw near the ice-boat!" began Harry. "It must have
been him! Who was he?"