It was not the horizontal trunk of a tree with a branch projecting from it at right angles.

It was nothing that was natural or had grown.

It was plainly the work of man.

It was a machine.
At first the captain thought it was made of wood, but afterwards he believed it to be of metal of some sort.

The horizontal portion of it was a great cylinder, so near the bottom of the lake that he could almost touch it with his hands, and it was supported by a massive framework.