Maybe it's been washed off from some ship, or gone adrift from shore.

Put her hard down, Mr.
Tomlinson, for it just so happens that I am in need of a boat at present." Half a minute later the _Golden Rod_ had swung round and was running swiftly down towards the black spot which still bobbed and danced upon the waves.

As they neared her they could see that something was projecting over her side.
"It's a man's head!" cried Amos Green.
But Ephraim Savage's grim face grew grimmer.

"It's a man's foot," said he.

"I think that you had best take the gal below to the cabin." Amid a solemn hush they ran alongside this lonely craft which hung out so sinister a signal.