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Proceedings of the Royal Society 
natural silt producers, which never fail to generate a constant and 
large supply of matter for deposit. 
V. Natural Limit to the Erection of Walls. 
Given the above features, the only known expedients for counter- 
acting the evil of the reduced amount of scouring power, consequent 
on siltage, are those proposed by my brother, Mr D. Stevenson, 
about forty years ago — first, by continuing the training walls 
down to the sea, and thus concentrating what yet remains of 
the reduced scouring power upon the lower channel of the navi- 
gation; and, second, by the erection of breakwaters in deep water 
to shelter the bar from the beating of the sea upon its surface, and 
thus to neutralise, if not to annihilate, these great originators of 
silt. If, owing to the local circumstances of the case, both of these 
plans be impracticable, we may almost regard these circumstances 
as presenting a fifth and serious element of danger. 
VI. Amount of Matter in Mechanical Suspension. 
But, again, instead of arguing from principles, the amount of 
matter in suspension may be ascertained by direct observation, and 
will thus form another definite element in the comparison of rivers. 
VII. The. Existence of a Wide Land-locked Estuary. 
And last, and one of the most dangerous of all, the existence of a 
wide land-locked estuary , as already referred to. 
Such are the criteria which we shall now proceed to apply to the 
comparison of the river Mersey with six rivers which have all been 
improved by training walls. 
In three of these — the Nith, Lune , and Rihhle — there have been, 
as we have seen, great accretions ; in the other three — the Clyde, 
Tyne , and Tees — although training walls have been also long since 
employed, there have been hardly any or no deposit. 
Let us look first at the three last cases, as to which it was very 
strongly and pertinently urged before Parliament that no deposit 
has ever taken place, although training walls have been in existence 
for many years. 
Now it is not surprising that the introduction of training walls has 
