Was it imagination, or some sort of optical delusion that made the tip of the huge berg seem to come lower and lower, then draw back again as if making a ceremonious bow like a dancing-master?

Jack gasped, and opened his lips to cry out, but thinking better of it restrained the temptation.  They could not get away until the repairs were complete.  At the same time, while trying to make himself believe he had magnified the thing, he was conscious of a louder grinding noise than any heard up to that moment.

Tom was putting the finishing bolt in place.  A few more efforts and he would be able to announce that his task had been completed.  Jack became conscious of a peculiar undulating movement to the ice under his feet.  It was just the same as he could remember experiencing when on skates, and going at full steam over a thin section of ice that must have easily broken under his weight only for the speed with which he crossed over.

Was the ice floe about to break up?  Would it result in several smaller sections separating from the main stem, none of which might be of a size to allow them sufficient room for making a start?

The thought alarmed Jack.  He also knew that undoubtedly any movement to the pack ice must be caused by some action of the giant berg.  Was that mountain of ice about to take the plunge at last, and turn over, its base being eaten away to such an extent that the whole had become top-heavy?

Once again did Jack turn his startled eyes to the left.  He could not get it out of his mind how terribly suggestive that “bow” on the part of the berg had been.

There it was, coming again!  Perhaps the wind had grown stronger since they dropped down upon the ice, and was adding its force to the action of the waters.

Jack found himself unable to hold in any longer.  If such a dreadful peril hung over them it was time his companions knew the need of haste in getting free from that doomed field of ice.  So he put all doubts behind him and gave tongue.

“Hurry, hurry, Tom!  The iceberg is acting queerly.  It’s tottering as if ready to roll over on us!  Don’t you see how it acts, Tom?”

CHAPTER XXII

WHEN THE ICEBERG ROLLED OVER

Fortunately Tom had everything ready for an immediate start, acting under orders, Jack and Beverly having previously changed the position of the big plane, so that it now faced the run taken when landing.

This brought the wind back of them; but that would be an asset rather than a detriment.  They had also gone hastily over the course to make absolutely certain there was no break, or other trap, which might give them serious trouble.

“Jump aboard, both of you!” cried Tom, still keeping his head—­a lucky thing, since to get “rattled” in such a crisis might prove fatal.

The beating of the engine and the whirr of the propellers announced that they were off.  On the comparatively smooth ice it was easy to make a start unassisted by mechanics or hostlers.