"Oh, it's easy to run," said Mollie, calmly. "I understand it now. Really, it's very simple."
The girls took turns steering, for the boat was not going very fast, on account of the condition of the ice. Once or twice there were booming noises, like the sound of distant cannon.
"What are those?" asked Amy, with a start.
"The ice cracking," explained Betty. "It isn't anything. It often happens on a big surface, and we're on a wide part of the river now."
They went on for a mile or so, until Mollie suddenly clutched the arm of Betty, and cried:
"Look—there's open water ahead!"
"That's right," agreed Betty, as she quickly shifted the helm. "We don't want to plunge into that," for the water looked black and treacherous in contrast with the white ice about it.
They headed for their camp. The sound of the cracking ice became oftener, and more than once Betty looked a bit apprehensively at Mollie. But they tried to conceal their growing uneasiness from Grace and Amy.
Suddenly there came a sharp report, louder than any that had gone before, and, involuntarily, Mollie raised the spiked wheel. The ice boat slowly lost headway.
"Don't stop! Don't stop!" cried Betty. "Keep on!"