The next morning they encounterd another mystery when?

From the text:

"In the morning, however, as soon as it was light, I went upon deck, and found all the sailors busy on one side of the vessel, apparently talking to some one in the sea. It was, in fact, a sledge, like that we had seen before, which had drifted towards us in the night, on a large fragment of ice. Only one dog remained alive; but there was a human being within it, whom the sailors were persuading to enter the vessel."