"No wonder you are surprised," the silversmith said; "it was a piece of rare good-luck that my father hit upon it. A map that he had showed him that in the old days, before there were any houses on this side of the river, a narrow branch left the stream some hundred yards above the position of his house, made a circuit and came into it again as much below. He inquired among some old men, and learned that they had heard their grandfathers say that they knew that at some time or other this stream had been built over when Paris began to grow in this direction. After he had contrived this apparatus that you see, which is worked by a heavy counterpoise in the wall, he began to dig, and a foot below the surface came upon an arch of brickwork, so my father concluded that his house was exactly over the old stream.