The most generally successful mode of improving rivers of smell and irregular volume is, however, their conversion into still water, or canal navigation., by the construction of locks, waste weirs, and moveable damn, The positions which are the best fitted for the construction of the locks are upon the subsidiary channels formed by the islands in the mid stream of the river ; and, unless some peculiar local circum stances should intervene, it is customary to form the dam and waste weir near the head of the smaller branch, and to place the lock at the lower end of the island ; so as, in fact, to convert the smaller branch into the navigable channel, end to leave the wider branch as the storm or waste water passage, or gout, as it is technically called. The prin ciples of construction of of course the same in these cases a4 in canals (Cases]; and perhaps this portion of the subject may be dianieled, by that the only special works required for river navigationa are th by means of which sudden floods are allowed to peas ; for especial care must be taken to guard against the formation of cataracts on the down side of the lock gates. The style of move able dam used upon the upper Seine appears to be one of the most 'satisfactory of any yet tried, and it might advantageously be adopted on menysof our colonial torrential rivers. Their mode of construction Ina been described in a special treatise, by their author, M. Poirac, lea a brief account of them will be found in the ' Aide3lemoire of 31ilitary Sciences,' article, River Navigation.
The position of the towing-path is, after all, one of the most important parte of the operations cenoected with the improvement of the navigation of an inland and non-tidal river; and it maybe stated that this path should be placed, wherever it is possible no to do, by the side of the deepest water, and immediately upon the banks of the river. The theoretical condition's required for such paths aro, that they should be exposed to At few impediments to the passage of the tow-ropes as may bo, and that the direction of the haulage should not be too oblique. It is desirable to keep the paths under the wind, in order to avoid the action of the latter upon the boats ; and when bridges are erected, to carry the path. under the land arches, in order to avoid the necessity for seating off the ropes. The width of the principal path should be about 12 or 13 feet; and if, In consequence of the existence of any rapid or current, It should be necessary to form a second path, so as te allow another rope to be thrown out to keep the boat's head well in the centre of the stream, the second path may be made only 6 feet 6 inches wide. The height of the towing-paths should be made such as to allow of their being overflowed directly the river rises to such a height, during floods, as to render the navigation dangerous ; or per haps a side general rule would be, to keep the top of the path only 2 feet above the ordinary winter flood line. Great attention is required in the maintenance of the banks and of the roadway of the towing path, especially if a rapid navigation be maintaiteel ; the banks in this cam should he pitched, and the surface of the roadway macadamised.
Secondary streams or small affluents should be bridged over, so as to avoid the necessity for casting off the tow-ropes on approaching them.
In rivers with light and easily moved beds, and which are subject to violent floods, the navigable channel will les found to min between numerous islands, and occasionally to shift from one to the other bank of the stream. In such cases it frequently becomes necessary to rectify the bed by forming a new channel, or to fix the current in its original direction by works which should confine it in one of its previous channels. This class of operations is one which requires con summate skill, for the nature of the foundations is such as to render it more than usually difficult to contend with the underinining action of the stream itself, and any injudicious diversion of the current will in all probability endanger the banks of the ricer for a great distance. The most simple rules of a general nature hitherto applied in settling the dimensions and directions of new channels may be stated as follows : it being always observed that local circumstances may modify their application in a very serious manner. Firstly, with regard to the conditions of flow in the channels, it is known that the velocities vary in the inverse ratio of the cube roots of the widths; and secondly, that the cubes of the depths are in the inverse ratio of the widths ; in practice the capacity of the new channel should be made equal to that of the ancient one, but the capacity in question should be obtained, wherever it is possible, by increasing the depth at the expense of the width. All obstacles to the navigation, in the shape of trees, rocks, shallows, (ke., should be removed ; and the new channel should be con nected with the old one by curves of considerable radius. The banks of the new channel on the side opposite to the towing-path should be made in such a manner as to allow the water to flow into the bye wash as soon as the floods bring down an abnormal quantity of water ; and it is very seldom, in ordinary river-barge navigations, that the crowns of the dams the bye-washes are raised more than two feet above the fall summer level of the stream. Excellent examples of the :node of constructing these lateral banks are to be found in Sineaton's works on the Carron, Telford's on the Weaver, and D. Stevenson's on the Ribble, though in some of these cases the tidal action intervened; perhaps the best treatises to be consulted on this subject are, however, Memoirs on the Rectification of the Bed of the Midouze; in the Annales des Fonts et Chaussdes,' 1831 ; a`Memoirs on the Defence and Improvement of the Navigable Channel of the Rhine,' by M. Defoutaine, in the same publication for 1833; in Sganzin's Coors de Construction;' and in Brook's, Calver's and I D. Stevenson's treatises on River and Canal Engineering.