Harriet and Jane crept to the door of the houseboat and peered out.

As they did so a second splash startled them.
This time they saw the stone.

It was a good-sized rock.

It fell some feet below the rear end of the "Red Rover." Some one was sounding the thick growth there.

Who it was, they discovered a moment later.
"There's water down there, but it's shallow.