The treatises last mentioned are, it is to be observed, more particu larly composed with reference to the engineering works required for the improvement of the tidal or of the salt-water portions of rivers ; but it may be added that, in all cases, the results of experience show that the most certain mode of regularising and deepening the bed of a river is by the establishment of continuous longitudinal einbankments. With respect to the tidal or marine parts of the streams, the only remarks to be made in addition to those previously applied in the course of this notice are, that so long as the rivers discharge into seas wherein the tidal action takes place without interference, the navi gation may be improved, and the depth of the channel maintained in an efficient state simply by the construction of lateral embankments, diminishing gradually in width according to the quantity of water flowing inland. Any sudden widening or contraction of the channel is injurious; but nature will eventually remedy the former disadvantage, for alluvial matters will be thrown down in the species of bay thus formed, and all that will then be required will be to keep the main channel open, in the precise direction required, by works of compara tively easy execution, In rivers like the Thames and Medway, the Seine, Rhine, ke., whore the flood tide is enabled, by the peculiar laws of the deep sea currents, to carry in more alluvial matters than the ebb can remove (and in discussing this question of the movement of alluvions in rivers, the difference in the specific gravities of salt and of fresh water must not be lost sight of), in such rivers there must always be a danger of the channels becoming gradually silted up. The proper course to be adopted in such cases is to combine an efficient system of dredging with a regularisation of the upper portions of the channel, so as to developo and to concentrate the scouring action of the tide and of the current to the utmost extent; but it must always be understood that rivers placed under the action of this peculiar set of laws can only be kept open by the exertion of great skill, energy, and enterprise. It
is true that hitherto little inconvenience has been felt in the Thames ; iri the Medway, however, a gradual shoaling up of. the tidal portion of the river has been observed ; the mouths of the Rhine are successively being -choked ; the mouth of the Scheldt is becoming less and less adapted for largo vessels; and the Seine has been shown to present this peculiar characteristic, namely, that even at Rouen the alluvial mud consists rather of materials brought in by the flood tides from the sea, than of materials brought down from the interior by the fresh water. It may also be added, that, as the height and volume of water in a tidal river depend far more on the quantity of the tidal stream than upon any supply from the interior, it is comparatively easy to calculate the height to which the water may under any circumstances rise ; and, when this has been ascertained, the banks on either side should be carried above the highest water-line ; where towing-paths are used in rivers of this description, they should, if possible, be formed on both sides. A tidal current of about two to three miles per hour is usually considered to be advantageous for the trade of a river; and it is to the extraordinary facilities offered by the tides of the Thames that we may attribute the singular form of barge used upon it. These barges are in fact little better than logs which float on the top of the tide, and they contrast strangely with the barges used on the Seine, Ithine, 1thDue, k-c., which are built in such a way as to offer the least possible resistance to the current they are obliged t encounter beyond the limits of the tidal action.
Much information on the subjects alluded to in t his article is to be found (in addition to the works already named) in Rol 'Lion's Mechanical Philosophy ; in Ellet's Mississippi and Ohio Rircrs ; in the Raccolta dei .41dori Italiani chi tra nano dr1m610 del argue ; Weibekinea Theoritische Practische Wasserbastkunst : De Prony's Nourelle Architecture Ilydran Gime ; D'Aubisson's Ilydrardique ; General Baird Smith's Reports, and his works on Irrigation. &c., &O.