one. Such beaches, however, do not travel, but gradually add to the surface of the land.
The following facts may also be appealed to in favour of the greater power of tidal currents:—a ship at anchor always rides with the tide, except in excessive wind; and then it is the wind, not the wind-wave, that veers round the ship. Fishermen's nets that break loose during a storm always float with the tidal current; so also do wrecks, timber, and the like: these being below or only a little above the surface of the water, cannot be driven by the wind; but if the wind-waves are more effective than the tidal currents, they ought to go with the former, not with the latter[1]. We also find that off-shore banks and shoals always lie with the course of the flow-tide currents, irrespective of the direction of the most prevailing or most effective winds[2].
It will be found that if the effective winds come in a contrary direction to the flow tide, all permanent sand and gravel banks, diverting the "invers" or mouths of rivers and streams, are due to the currents of the flow tide. Continued winds in a contrary direction may often temporarily pile up a bank; but as soon as they cease, the bank is rapidly carried away and disappears[3]. Many interesting facts may be learned from the study of wrecks on sea-beaches. If a ship is cast broadside on, and the wind and tidal currents are in the same direction, the beach inside the wreck is scooped out. If, however, the current and wind-waves are contrary, a bank is cast up behind the wreck, the greater accumulation of the sand being to
- ↑ Instructive experiments may be made with bottles so filled that they will float at, but not above, the surface of the water, also with corks. These will go with the flow of tide (though not with its ebb) across a whole gale of wind.
- ↑ In some places on the east coast of Ireland both the flow and ebb tide, as marked on the Admiralty charts, run obliquely across the off-shore banks. In such places, however, they are only surface currents, as there are undercurrents along the banks. It may be stated that the latter currents are caused by the banks; but if so, what originally formed the banks? As the banks are parallel to the flow-tide current in the channel of the Irish Sea, does it not appear probable that these undercurrents may be the edge of the mid-channel current?
The most prevailing and most effective winds are not necessarily the same. In the south of Ireland the most prevailing wind is from the S.W.; this wind sweeps across the lowlying S.E. portion, and on the S.E. coast is an off-shore wind that has little effect, the most effective winds coming from the S.E. and N.E.
- ↑ In the East Bay (Portland) there are two drifts, one northward from Portland towards Weymouth, and the other westward from Lulworth towards Weymouth. It has been suggested that these driftages are due to the bay being open to the full force of the wind from the S.E. If, however, we examine the tidal currents, we find that an offset incoming stream runs from Portland northward while the offing tide forms an on-shore tide in the eastern portion of the bay, which flows from east to west. The set of the offing tide on shore here must be very considerable on account of the long loop each hour's tide makes up the channel (see any co tidal map), the VII.-o'clock wave (Full and Change of moon) touching the shores at Portland Bill and Cape la Hogue at the same time that the end of the loop is further up than Portsmouth; while the loop of the VIII.-o'clock wave is still further out of proportion.
