MEETINGS OF SOCIETIES. 183 
posited as soon as it is beyond the influence of the waves ; the ebb 
tide, being unassisted by the waves, is unable to cope with the 
incoming sand, and thus we see, when the tide and waves are left 
to themselves, the tendency is to close the entrance altogether. To 
this is to be attributed the deplorable state of our river mouths in 
time of prolonged drought. This point is very torcibly brought 
out in the notes on the Admiralty charts, where it is stated that 
certain entrances are only open after a heavy fresh. The opinion 
that bars are mainly formed by the action of the waves is held by 
many leading authorities on the subject. Mr. David Stevenson, 
F.R.S.E., member of Council Inst. C.E., said (Minutes, Inst. C.E., 
Vol. XXXVL., p. 236):—‘“ It was now thirty years since he pro- 
pounded the theory—and at that time he believed he stood alone 
in holding it—that the bars of the rivers of the United Kingdom 
were due entirely to the action of the sea constantly heaving up 
sand and detritus, and that, but for some counteracting influence, the 
effect of that action would be to form a continuous line of beach 
across the mouths of navigable rivers and estuaries. The counter- 
acting influence to which he referred was that of the tidal and 
river scour. He might say that that theory, now 30 years old, 
had been fully confirmed by his subsequent experience. : 
It was further known that those bars were always worst after a 
prevalence of on-shore wind and heavy sea, and were best when 
the river.was in flood. . . . The waves were the true ‘ deposi- 
tors’ of the bar, the river was only an ‘excavator,’ and there would 
still be all the phenomena of a bar at the mouths of estuaries, 
although the river water did not bring down a single particle of 
suspended matter. . . . If his bar theory, asapplicable to tidal 
rivers, was right, it clearly followed that, if the pier heads were 
carried into water of sufficient depth to prevent the waves from 
acting upon the bottom with such force as would throw up sand- 
banks, it would be possible to secure permanently the depth ob- 
tained by extending the piers, because there would be no submerged 
beach thrown by the sea, and the alluvial matter in suspension 
mould be carried out with-the current.” . Mr. I. J. Mann,,C.E., in 
his valuable paper on the gemoval of river bars by induced tidal 
scour (‘‘ Engineering,” a XXVII., p. 108), remarks :—“* With 
reference to the bar at the mouth ot thie Liffey, the author has no 
hesitation in attributing its formation to the combined action of 
waves and current of the flood tide, the former stirring up and: 
keeping in agitation the fine sand of which the bottom of the bay 
is composed; the lower stratum of the water becomes therefore 
surcharged with sand, which is carried along by the tidal current.”’ 
Sir John Coode, remarking on the formation of bars, says (Minutes, 
Inst. C.E., Vol. LVIII., p. 130):—** They were formed almost en- 
tirely by the sea, some rivers illustrating this in Australia. At the 
Swan River, on the coast of Western Australia, facing the Southern 
Ocean, with very little tide, there was a bar of the worst possible 
description ; while the Yarra, at Melbourne, which discharged into 
a sheltered embayment at the head of Port Phillip, though it had 
a rise of tide precisely the same as the Swan River (about 2 feet), 
had no bar, simply because it was in a sheltered position, and there 
was no heavy wave action to throw up the material to forma har.” 
The same remarks are equally true in the case of the Hawkesbury 
River, the entrance of which is sheltered, and thereis therefore no 
