90 Proceedings of the Royal Society 
that they would exclude one-twentieth of the scouring tidal power 
of the Tay. 
In the opinion, then, of Mr Walker, the first engineer of his day, 
and the one who carried out most of the later improvements of the 
river Clyde between Glasgow and Dumbarton, the loss of one- 
twentieth of the natural tidal scouring power at the Tay ought 
to he firmly resisted. But is this caution to be accepted, or should 
it be regarded as excessive % 
Turning to ascertained fact, the only one to which I can appeal 
is that observed at the Lune, for I have no means of knowing what 
has actually taken place at either the Mth or the Kibble. 
The loss of tidal scouring water due to the training walls at the 
Lune was found to be one-tenth , but from this loss no evil conse- 
quences appear to have resulted. We have then, by way of criteria, 
only two to guide us, viz., Mr Walker’s opinion that a loss of one- 
twentietli would prove injurious at the Tay, and the ascertained fact 
that a loss in the case of the Lune of one-tenth of the scouring power 
did no harm. 
Turning now to the case of the Mersey, and assuming, as we 
have more than sufficient reason for doing, that deposit of sand will 
take place up to the grass level in the land-locked estuary of that 
river, as it has done already in the kindred but less dangerous cases 
referred to, we have next to ascertain what would be the ratio of 
loss in that case. 
The total amount of deposit, as ascertained from actual soundings, 
would be about 460,000,000 cubic yards, measuring from the pre- 
sent bottom up to grass level, while the additional tidal capacity 
which will be simultaneously gained through dredging and scouring 
the improved channel will amount to about 14,000,000 cubic yards; 
and, deducting the one from the other, we find a balance of loss of 
446,000,000 cubic yards. 
But in order to judge of the effect of this loss, we must contrast it 
with the total amount of tidal water above the level of low-water 
spring tides, which at present enters the estuary from the sea at 
the mouth of the river, and which fills the tidal basin up to Run- 
corn. This amounts to about 733,000,000 cubic yards, to which 
must be added the average fresh-water drainage of the Irwell and 
Mersey districts, which I compute at about 9,000,000 cubic yards 
