"Probably for the same reason that there are mosquitoes," said Betty. "It's so we will appreciate nice people all the more. But don't worry, Grace." "Are you girls going out in that boat?" asked Mr. Franklin as he started back toward his cabin. "Yes. Why shouldn't we?" inquired Mollie, for she saw a look of concern on his face. "Well, you'll be all right if you stay around here, but the ice is breaking up below and above you, on account of the thaw. It won't be safe to go too far, or you'll meet open water. Be on the lookout." "We will," promised Betty. "We're only just going out for a practice spin by ourselves. It will surprise the boys." She did not realize what a surprise she and her chums were to get before long. After one or two ineffectual attempts the girls got the motor running. Then, looking to see that all was clear, Betty, who was at the helm, gave the word for Mollie to lower the toothed wheel, which engaging on the ice, would move the craft. At first there was only a shower of soft and rather watery ice. The surface was too "mushy" to enable the teeth to "bite." "Harder! Push down harder!" directed Betty. Mollie did so, and then, after hesitating a second as if uncertain whether or not to go, the _Spider_ moved off, gradually acquiring speed. "Oh, this is glorious!" cried Grace as she sat well forward and breathed in deep of the fresh air. "Betty--Mollie--you are wonderful!" "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!" "But it may be dangerous!" "It will be more dangerous to stand still! Don't you know that a moving body has a better chance over thin ice than one standing still? Keep going, Mollie, and head for shore!" "Oh, I'm sure something is going to happen!" cried Amy. "Nonsense, be quiet!" urged Betty. "Grace, give her a chocolate! Mollie, lower that wheel again."