The pile must be considered as a whole, and it won't float until there is water enough to float the whole.

The bottom logs can't float while those above them are clear out of water, if their weight rests on the bottom logs, as it does in the drift-pile.

You see when you put anything into the water, it sinks until it has displaced a bulk of water equal to its own weight, and then stops sinking.

In other words, that part of the floating thing which goes under the water is exactly the size of a body of water equal in weight to the whole thing.

If a log floats with just half of itself above water, you know that the log weighs exactly the same as half its own bulk of water, or, in other words, that its specific gravity is just half that of water.