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:
"Ice or—"
"There it goes again!" exclaimed the Doctor.
This time the blow was heavier. It sent them against the side of the compartment.
"Ice beneath the ocean? Impossible!" exclaimed the Doctor. "Must be rocks!"
Another blow hurled them in the opposite direction. Both realized the gravity of this new peril. If one of these blows caught the craft squarely it would crush the sub like an egg-shell.
But the boat was slowing up. There was hope in that. Dave, attempting to look out of one of the portholes, was thrown to the floor by another shock. And this time the craft seemed to have stuck, for she did not move.