Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/60

34 windward or leeward respectively, if the tidal current or wind-waves have the greater power. In the latter case, if the cargo "breaks loose" it will be carried, if not very heavy, by the wind-waves; heavy cargoes, such as coal, if the gale be continued, will be more or less carried to leeward, but eventually will go with the tidal current; a cargo of green timber or "pit wood," as it floats just at the surface, is usually stranded to windward, having been carried by the tide against the wind-waves. "While examining into the relative driftage powers of wind-waves and tidal currents, there is one kind of the latter nearly always ignored; that is, the current due to the on-shore set of the offing tide. A wreck may be off the coast and the wreckage drifting, say northwards, across a gale from the east or north-east; it will be carried in that direction for half tide (three hours); but after the offing tide sets in, it will be carried to the shore. This on-shore driftage is usually supposed to be due to the wind; and fishermen explain it thus:—"The wind has no force till half the tide is gone."

Second Point.—"Flow and ebb of the tide are equal and opposite, and therefore counteract one another's effects." Let us examine into certain effects of the ebb and flow of the tide which can be distinguished. On a calm day, when there is not a ripple on the water nor a perceptible ground-swell, if the tide is ebbing it imperceptibly drops away from the shore, leaving every thing undisturbed. If, however, it is flowing, the strand under similar circumstances gets saturated with water before the waves go over it, which causes each particle to be more or less buoyant; so that even in low neap tides, where the current is least, fragments of considerable size will be moved, although only an inch or two. During spring tide, the in- coming tidal current is greatest; therefore it might be supposed that the outgoing current also would be greatest at this state of the tide; yet under such circumstances we find the water drop away from the shore without disturbing those portions of the beach that are never uncovered except during "spring tides." This of course only refers to ordinary coast-lines; as in "narrows," and round certain points there are ebb-tide currents which effect considerable local driftage. Further- more, in estuaries and bays the mud and sand banks always indicate, from their shapes and positions, that they are more due to the flow than the ebb of the tide. This is to be seen even in estuaries into which large rivers flow, and in which the ebb tide is there augmented. It may also be pointed out that the margin of the flow tide, no matter how calm the water may be, is always more or less "dirty" (with fragments of seaweed and such like); but this dirt is left behind when the tide ebbs. The flow tide when it comes to a stranded obiect pushes it before it; but a slight resistance will prevent a similar object from being dragged out by the ebb.

When the flow tide is helped by the wind its effects are considerably increased; but wind-waves do not proportionally augment the ebb tide. This can be seen during a moderate gale of wind, when waves are breaking on a beach. If the tide is flowing, the