like manner the offing tide on the nodal lines begins to ebb at half-ebb on shore—which is the relation between the offing tide and the onshore tide, known to sailors as "tide and half-tide"[1].
Driftage of Sea-beaches.
Now as to the driftage of sea-beaches. In Prof. Prestwich's paper on the Chesil Beach, and the discussion that followed thereon, certain points were raised. Of these the first and principal one seemed to be that wind-waves have greater driftage power than the tidal current. On this point it would appear, from the report, that Prof. Prestwich and nine of his critics believe in the greater driftage-power of wind-waves, one of the latter in wind-waves and tidal currents combined, while only two believed in the superior power of the tidal current over wind-waves[2]. As to the second, the author of the paper goes so far as to state that the currents of the flow and ebb tides neutralize each other's effects. The third point brought forward was that a current carried the smallest fragments furthest.
First point.—From what has been stated it is evident that the following is at variance with the opinions of many eminent men. In another place[3] I have attempted to show that on the Irish coast the current due to the flow of the tide has greater drifting powers than wind-waves, the driftage due to the latter, if contrary to that of the incoming tide, being eventually carried back again, and that the maximum driftage results from a combination of the tidal current and of waves caused by a wind blowing in a similar or nearly similar direction to the flow of the tide.
Wind-waves make a great appearance; but if we look for the results, they seem to be very like the noisy fussy worker, who is "all show but little work;" while the tidal current may be compared to the quiet steady man who has most to show at the end of his day's work. Wind-waves can throw up beaches; but it would seem that, unaided, they cannot cause them to travel. In the delta of the Rhone (Ansted "On the Lagoons and Marshes of the Shores of the Mediterranean," Proc. Inst. C. E. vol. xxviii., read February 16, 1869) the wind-waves throw up beach after beach. So also on the north coast of Wales, where we find series of "storm-beaches" consisting of beach after beach, each outside the preceding
- ↑ In the English Channel and the Irish Sea, the surface tidal currents are known from the investigations carried on by the officers of the Royal Navy; there are, however, under tidal currents that have never been investigated. They are known to exist; but their laws and relations one to another are perfectly unknown. On the subject of an inquiry like the present, they must exercise a vast influence; but unfortunately we cannot refer to them.
- ↑ Of the ten advocates for the wind-waves, all seem to have studied principally on the south coast of England, where, as the prevalent wind-waves and the "flow" of the tide are in the same direction, one force may easily be mistaken for the other; while of three who believe in the tidal current, two have studied the wave on the south, and the other on the west coast of England. One of the other commentators, who has studied on the east coast of England, does not give any definite opinion.
- ↑ "Tidal Currents versus Wind-waves," Proc. Roy. Irish Academy, ser. 2, vol. ii. p. 443, and Geol. Mag. dec. ii. vol. iii. p. 83.
