of Edinburgh, Session 1884 - 85 . 
85 
to keep open either the bar or the lower channels, then the naviga- 
tion will be ruined. 
This possibility once admitted, that the benefit of the application 
of training walls really depends on the relative proportion of the 
good and evil influences, it becomes necessary next to consider in 
more detail the natural features which I believe tend to the forma- 
tion and deposit of matter in suspension, and which ought, there- 
fore, to be regarded as dangerous in the application of training walls 
to any river where there is a wide estuary. These characteristics 
have, so far as I know, not been before referred to in connection 
with the subject now under consideration. 
In future, then, I consider it essential that in every case the fol- 
lowing characteristics ought to be fully inquired into and taken into 
account before training walls are resorted to : — 
I. A High Rise of Tide. 
Because if there be matter in suspension in the sea (due to the 
action of the waves on the bottom when they reach shoal water), 
the higher the rise of tide the greater will be the amount of solid 
matter that will be brought from the sea. 
II. A Rapid Tidal Flood Current. 
Because, from erosion of the sides and bottom, the more rapid 
the current the greater the amount of sandy and silty matter that 
will be disturbed and carried up the river. 
III. Tidal Bore. 
Where there is a tidal bore with a cresting or breaking head, 
which, from its necessarily acting in shallow water, and with a very 
high velocity, greatly cuts up the bottom and sides of, the channel, 
so as to set free a large amount of matter. 
IV. A Bar. 
Because if there is either a bar or extensive sandbanks, with 
heavy breaking waves, at the mouth of a navigation, large quantities 
of matter are set loose, which may be carried inward by the flood 
tide, so that a bar and the adjoining sea shores may be regarded as 
