And he was right.  The crew of christianized natives were soon alongside, very humble in their apologies, and very anxious to assist in undoing the damage they had wrought.

“Have we any extra steel plate?” asked the Doctor.

“Yes, sir.  Have to be shaped, though,” replied Dave.

“Can we do it?”

“I think so, on shore.”

“All right, then.  Get these natives to give us a hand and we’ll go on the sand-bar for repairs.  Bad cess to the whaling industry of the Eskimos!  It’s lost us a full two days, and perhaps the race!  But we must not give up.  Things can happen to airplanes, as well.”

It took a hard half-day’s work to bring the craft to land, but at last the task was done and the mechanics were hammering merrily away on the steel with acetylene torch sputtering, and forty natives standing about open-mouthed, exclaiming at everything that happened, and offering profound explanations in their own droll way.

CHAPTER XV

THE MYSTERY CAVERN

Once their craft was repaired, the submarine party pushed northward at an average rate of ten miles an hour.  It was two days before any further adventure crossed their path.  But each hour of the journey had its new thrill and added charm.  Now, with engine in full throb, they were scurrying along narrow channels of dark water, and now submerging for a sub-sea journey.  Now, shadowy objects shot past them, and Dave uttered a prayer that they might not mix with the propeller—­seal, walrus or white whale, whatever they might be.  In his mind, at such times, he had visions of floating beneath the Arctic pack, powerless to go ahead or backward and as powerless to break through the ice to freedom.

Wonderful changing lights were ever filtering through ice and water to them, and, at times, as they drove slowly forward, the lights and shadows seemed to have a motion of their own, a restless shifting, like the play of sunlight and shadow beneath the trees.  Dave knew this was no work of the imagination.  He knew that the ice above them was the plaything of currents and winds; that great cakes, many yards wide and eight feet thick, were grinding and piling one upon another.  Once more his brow wrinkled.  “For,” he said to himself, “it may be true enough that the average ice-floe is only twenty-five miles wide, but if the wind and current jams a lot of them together, what limit can there be to their extent?  And if we were to find ourselves in the center of such a vast field of ice with oxygen exhausted, what chance would we have?”

Dave shuddered in answer to the question.

He was thinking of these things on the eve of the second day.  They were plowing peacefully through the water when, of a sudden, there came a grating blow at the side of the craft.  It was as if they had struck some solid object and glanced off.

“What was that?” exclaimed the boy.  He cut the power, then turned to the Doctor: