To one of these cakes of ice, towards which a boat from the schooner was making its way, the captain directed Mark’s attention.  On this cake, which was about as large as a dinner-table, stood a man anxiously watching the approach of the boat.

“What I can’t understand,” said the captain, “is where he ever found a cake of ice at this time of year strong enough to bear him up.”

“Who is he?  How did he get there, and what is he doing?” asked Mark, greatly excited.

“Who he is, and how he got there, are more than I know,” answered “Captain Li.”  “What he is doing, is waiting to be taken off.  The men on the tug sighted him just before you came on deck, and sung out to me to send a boat for him.  It’s a mercy we didn’t come along an hour sooner, or we never would have seen him through the mist.”

“You mean we would have missed him,” said Mark, who, even upon so serious an occasion, could not resist the temptation to make a pun.

By this time the boat had rescued the man from his unpleasant position, and was returning with him on board.  Before it reached the schooner Mark rushed down into the cabin and called to his parents and Ruth to hurry on deck.  As they were already up and nearly dressed, they did so, and reached it in time to see the stranger helped from the boat and up the side of the vessel.

He was so exhausted that he was taken into the cabin, rolled in warm blankets, and given restoratives and hot drinks before he was questioned in regard to his adventure.

Meantime the schooner was again slipping rapidly down the broad river, and Mark, who remained on deck with his father, questioned him about the “river’s breath,” as he called the clouds of steam that arose from it.

“That’s exactly what it is, the ‘river’s breath,’” said Mr. Elmer.  “Warm air is lighter than cold, and consequently always rises; and the warm, damp air rising from the surface of the river into the cold air above is condensed into vapor, just as your warm, damp breath is at this very moment.”

“But I should think the water would be cold with all that ice floating in it,” said Mark.

“It would seem cold if we were surrounded by the air of a hot summer day,” answered his father; “but being of a much higher temperature than the air above it, it would seem quite warm to you now if you should put your bare hand into it.  We can only say that a thing is warm by comparing it with something that is colder, or cold by comparison with that which is warmer.”

When Mark and his father went down to breakfast they found the rescued man still wrapped in blankets, but talking in a faint voice to the captain; and at the table the latter told the Elmers what he had learned from him.