WAVES and TIDES possessing in many respects the same charac ter, it baa been thought proper to state in one article the phenomena and the theories of both.
From the earliest times the periodical risings and fallings of the tvatets on ceasts or In rivers have been noticed ; and the recurrences of the phenomena depend so obviously on the positions of the moon and sun, that the influences of thou celestial bodies in producing them have ever been arraigned as their cause. The tide appear as a general wave of water which gradually elevates itself to a certain height, then u gradually sinks till its surface la about as much below the medium level as it was before above it: from that time the wave again begins to the; and this reciprocating movement of the waters continues con Manny, with certain variations in the height (with respect to the mean level), and in the tunes of attaining the maxima of elevation and depression.
Ordinary waves arc produced by any cause which disturbs the equi librium of the particles of a fluid. Thus, a stone suffered to fall Into water at rest gives rise to a series of concentric eireidar waves extend ing to a great distance from the place where the stone falls; and in a canal the fall of a body of water from a level above that of the general sturface will produce a zories of waves advancing along the canal. Waves aro oleo produced by suddenly pressing a solid into water, or by suddenly withdrawing it from thence ; and a single wave may be caused by partly immersing a solid body in water and moving it quickly, for a time, in a horizontal direction. The inequalities of the pressure of the air on the surface of water, whether at rest or in motion, when a gentle wind is blowing, will produce ripples; and if the action bo continued long, the ripples, at a certain distance from the place of their origin, become considerable waves. In the open seas the heights of the waves depend on the force of the wind; but In confined situations both the heights and forme of the waves are affected by the resistance of the bed, by reflections from the shores, and other eircumitances. When warts aro former] by wind blowing from the land, each wave-summit presems constantly the same height ; but the heigtda go on increasing with the distance from the slime.
'Wind-wawa appear generally to be of a cycloidal form : their sum mits have a gentle curvature, while the height been a =all propor tion to the length in the direction of the motion ; but as the height increases, the summit becomes more,acute, and assumea the form of a ridge ; and when this becotnes too sharp for the preservation of equi. librium, the force of the wind acting horizontally near the top break. it into foam or spray (col. 783 of the present article. The nature of this spray itself has been noticed in WATEDYALLAq As waves advance towarde a shore, the water becoming less deep, le resistance of the bed of the sett causes their length. to diminish, and et the same time
their heights to inereaae, so that the front of the wave becomes steep ; and the motion of the upper part, towarde the land, being more rapid than that of the lower part, it follows that the summit is carried beyond the base ' • and, falling forward, there is produced whet is called a surf. Tho breaking of leaves over a sunk shoal depends chiefly on a like cane.
The surface of the sea often presents very complex phenomena. It may happen, for example, that while a long swell resulting from some distant storm is advancing in one direction, a breeze will produce a ,series of waves moving in the direction of the wind ; and a second breeze springing up in another direction will produce a new seders, which will become mixed with the former without destroying them : a third gale may also produce a system of waves intersecting the other Eystems. When a breeze has been blowing for some time from a cer tain quarter, and afterwards changes to the opposite, two series of waves may bo seen moving in contrary directions ; and if the waves aro nearly of equal lengths, the vertical ordinates at the crest of the corn. pound wave will be equal to the alum or difference of the ordtuatea of the simple waves, according as the crests aro coincident, or fall in each other's intervals. These phenomena are indicated in the first and second of the subjoined figures, where a 1) represents the first undulation and a' b' the second, the straight lino A s being the horizontal surface c f the water when at rest. Again, when there exists a third system of waves, caused, for example, by reflection from a coast, so as to be parallel to the two former systems, the combination of the three 'systems has been observed to produce a compound wave of the form represented in the third figure. Report of the Committee of the British Association on 1'aves.) The wind-waves of the sea do not extend to considerable depths. From the experiments made by the committee appointed by the British Association in 1836, it was found that with a depth of water equal to 12 feet, waves 9 inches high and 4 or 5 feet long did not affect the water at the bottom. Waves from 30 to 40 feet long, oscillating at intervals of 6 or 8 acconds, produced sorne effect,but much less than near the surface : and it was ascertained that, in waves produced by the wind on the surface of a deep sea, the velocities were not a direct function of the depth. In a storm the sea is probably tranquil at the depth of 200 or 300 yards. The most satisfactory measuretnenta, those of the lath Rev. Dr. Scoresby (' Report of British Association; 1850, Journal of a Voyage to Australia'), give, for the height of the highest waves, 43 feet, from bottom of trough to crest.