Tidal waters are frequently charged, more or less, with silt, obtained from the shore or from shallows near it, either by currents or tidal waves sweeping along it, or by the action of ordinary waves. Tidal waters flowing up and down the lower portions of rivers render them to an enormous degree more capable of carrying navigation and, especially if they become enlarged and form estuaries, more capable of being altered by training works. A tide-gauge is constructed on the same principle as a self-registering stream-gauge. The rise and fall of the water are reduced, by mechanism, to a convenient range, and are recorded on a band carried on a drum, which is caused to revolve by clockwork. Another kind, which depends on the use of an inverted syphon filled with air and a syphon of mercury, is described in _Min. Proc. Inst. C.E._, vol. clxiv. [Illustration: FIG. 66.] 2. =Tidal Rivers.=--Let A B (fig. 66) be the surface of the lower part or mouth of a river, supposed to be of uniform width, and let B be the mean sea-level. As the tide rises to D the water of the river is headed up and assumes the line A D. When the tide falls to F there is a draw, the river surface taking the line A F. If the rise of the tide B H is so great that the discharge of the river cannot keep pace with it, so as to fill up the whole space between A and H to the level of H, there will be a flow of sea water from H to some point M, and of river water from A to M. The point M will be lower than A and H. If the tide now turns and the water-level H begins to fall, there will still be a flow along H M. For a brief period it will be due to momentum, but it will continue until, by the rise of the water-level at M and the fall at H, the surface has assumed the form indicated by the dotted line A N J. While this is happening, the point corresponding to M--where the concave curve of the upland water meets the convex curve of the tidal water--rises higher and shifts seaward. The character of the two curves remains the same, but they become flatter and the surface N J nearly level.