When the water rises say two or three feet, it will be above the level of the lower logs, and these would float away, if they were free, because their specific gravity is less than that of water.

But there is twenty feet of other timber on top of them, and its weight must be added to theirs.

The water displaced is exactly equal to their bulk, while the weight is many hundred times greater than theirs.

Do you understand ?" "Yes, I think I do.

You mean that the water must come high enough to pretty nearly cover the whole drift-pile before any of it can float." "Yes.