34 SAVING TIME IN NATURAL OPERATIONS.

lever, the action of the stream upon the wheels willkeep up a perpetual succession of blows. The sharp-pointed shoe striking upon the rock at the bottom,will continually detach small pieces, which the streamwill immediately carry off. Thus, by the mere actionof the river itself, a constant and most effectual sys-tem of pounding the rock at its bottom is established.A single workman may, by the aid of a rudder, directthe boat to any required part of the stream ; and whenit is necessary to move up the rapid, as the channelis cut, he can easily cause the boat to advance bymeans of a capstan.

(39.) When the object of the machinery just de-scribed has been accomplished, and the channel issufficiently deep, a slight alteration converts theapparatus to another purpose almost equally advan-tageous. The stampers and the projecting pieces onthe axis are removed, and a barrel of wood or metal,surrounding part of the axis, and capable, at pleasure,of being connected with, or disconnected from theaxis itself, is substituted. The rope which hithertofastened the boat, is now fixed to this barrel; andif the barrel is loose upon the axis, the paddle-wheels make the axis only revolve, and the boatremains in its place: hut the moment the axis isattached to its surrounding barrel, this begins to turn,and winding the rope upon itself, the boat is graduallydrawn up against the stream; and may be employedas a kind of tug-boat for all the vessels which haveoccasion to ascend the rapid. When the tug-boatreaches the summit the barrel is released from theaxis, and friction being applied to moderate its velo-city, the boat is allowed to descend.