In the lower parts of the courses of rivers discharging their waters into the ocean, and even to some extent in those discharging into tidal inland seas, a very marked interference takes place in the conditions of the currents, in consequence of the periodical tidal action. The tides themselves are produced by the periodical elevations and depressions of the level of the sea, caused by the combined action of the sun and moon, and under such circumstances they are naturally of greater amplitude on the shores of the larger bodies of water ; on the Pacific, for instance, they are greater than they are upon the Atlantic; upon the latter, again, the tides are greater than upon the Baltic or tho Mediterranean. The rising tide is called the flood ; the falling tide, the ebb ; and the returns of the tide take place twice in each con secutive interval comprised between the returns of the moon to the upper meridian. The mean length of this interval is P03505 day, so that the mean interval between two successive high waters is 0'517525 day, and the mean time of low water divides this interval again. into two nearly equal portions. As in the case of all quantities susceptible of a maximum and a minimum, the increase or the diminution of the tides towards their limits is proportional to the squares of the time elapsed between the high and low tides. The height of the full tide on the sea-shore varies, normally, every (Lay according to the phases of the moon ; it is greatest at the syzigies, and least at the quadra tures ' • but it is to be observed that in every place there is a species of retardation in the period of the high tide, as compared with the exact astronomical periods of the moon's changes, which is technically known by the name of the vulgar establishment of the place, and which depends upon laws to be noticed under TIDES. The rise of the high tides is usually proportional to the fall at low tides, but there is au irregularity between the heights of the two titles of the same day ; the rise and fall at the syzigics is about double those of the quadratures, the former being called the spring tides, the latter the neaps ; but it may be added, that the former are much increased when the moon is in perigee. An augmentation in the rise of the tide also takes place when the sun's declination is zero, or at the period of the equinoxes ; and the greatest normal tido occurs when a new or full moon happens to occur near the equinox, when the moon is in perigee ; and her action would be still further increased if her node should happen to coincide with the perigee. The other causes of the variations in the height of the tides may be neglected for the present, with the exception of those which are likely to affect their conditions in and upon rivers.
Thus, in cases where the tidal undulation is confined within a narrow gulf it becomes considerably increased in amplitude ; and its reflection from the opposite shore may also increase that amplitude, especially in the recesses of such coasts. It is on this account that whilst at St. Helena in mid Atlantic, the rise is only 3 feet, and in the Northern Atlantic it never exceeds 10 or 12 feet, it is in the Bay of Mont St. Michel, 46 feet ; in the Bay of Fundy, .50 feet ; and in the Wye, at Chepstow, 60 feet. Iligh winds, and especially the equinoc tial gales, when they coincide with the spring tides may also give rise to great irregularities in the amplitude of the tidal range, and instances have even been recorded in which the tides of the quadratures have been raised by the effect of storms to a greater height than those of the syzigies. The disturbing effect of storms is, however, the greatest
in was with a small tidal range; as upon the French shores of the Mediterranean, where the normal rise of the tides is only from 6 to 12 inches, it is thus often increased to 3 feet. The moat remarkable cause of disturbance in the range of the tides is, however, to be met with in the oceanic currents, and the manner in which they act varies in almost every case, producing some very singular interferences with the discharge of the upland waters, and with the deposition of the alluvial matters in the embouchures of rivers, whether the alluvions be brought from the am, or from the interior of the country.
For instance, upon the shores of England we find that in consequence of the set of the tidal currents in the offing, the bars at the mouths of the rivers between Land's End and Portland, point from N.W. to S.E., as a rough general rule; between Portland and Selsea Bill, the in-shore tide runs in a direction opposite to that of the tide in the offing, and the bars of the rivers also point from N.E. to S.W. On the Atlantic shores of Ireland, the bars point rather S.E. to N.W. ; they have the same direction on the western coasts of Scotland; whilst on the eastern coast of Scotland and of England, the bars upon the open coast point nearly from S.W. to S.E. In the mouth of the Thames two tidal currents meet, which have an origin separated by 12 hours' interval; po that not only are the day and night tides of that river equal in their amplitudes, but the alluvial matters transported by the tides are thrown down at the point where the velocities of the currents are dcetroyed, in banks which aro amstantly shifting. On the west coast of England again, there are in the rivers many instances of the double tide, arising from the reflection of the groat tidal wave from a pro jection situated at a point beyond their embouchures, on the line of advance of that wave ; and the conditions of the deposition of alluvial 'natters in those rivers are thus very different from tho conditions ordi narily prevailing. Nor is this the only technical peculiarity connected with these rivers ; because, in fact, the double tide is not so powerful as the normal tide for the knirpope of carrying forward a barge navigation; but in another sense it is more favourable for the operations of coin 111(.7Ce on a large scale, by reasun of its retainiug a great depth of water in docks and harbours, during the whole of the (so called) ilaclwide. At Southampton, this slack-tide lasts for nearly two hours ; on the French coast the same phenomenon occurs at Havre, and in the Orne, during about 1} hour In each tide. Great oceanic current's, wherever they exist, naturally produce analogous effects to these minor inter ferences with the progression of the tidal waves ; but of course upon a much larger scale. A study of the history of the deltas of such rivers As the Nile, Ganges, Indus, Irriwaddy, Amazonas, 3thshinippi, &c., be necessary for any one who desires to become acquainted with this important detail *Inflected with hydraulic' engineering. The actual records of the changes in the delta of the Rhine add of the Scheldt, would be of great iuterest and value also to the student ; because they have been more closely and more 'scientifically observed than the changes which have recently taken place In other riven; of similar magnitude.