THE ENTRANCES TO OUR RIVERS. 29 
extent of 10 feet. The following table gives a = account 
of the effect of these works since their completion 
Rate of _| 
Minimum Increase increase of ig tow on Bar 
Date. depth on Bar! Interval. | of minimum}! minimuny | at standard 
at low-water. depth de = per ieiewatey, 
ear. 
ft. in. years. ft. in in. ft. in 
1822 Sates eh tO ee 9 3 
1828 oe 6 6 3 3 6°50 226 
1833 10 6 5 1 0 2°40 23 6 
1856 13 0 23 3. 6 1°82 26 0 
1873 16 0 17 3 0 gg Bg 29 0 
An interesting —_ of the Dublin works will be found in Mr. 
I. J. Mann’s paper on “The Removal of River Bars by Induced 
Tidal Scour,” published i in Engineering, vol. 26, from which paper 
~ above table has been taken. Another history will be found in 
e “Minutes of the Institution of Civil Engineers,” vol. lviii, 
in a paper by Mr. J, P. Griffith, M. Inst. C.E. 
From these cases it will be noticed that there are certain fixed 
principles involved in each and all of them. The first is that the 
the ae out of the works. The third is that, whilst the flood- 
tide approaches from all directions, and any material it may bring 
in is deposited near the entrance, as soon as it arrives in compara- 
tively still water the ebb-tide and the upland waters having their 
forces directed and concentrated in a certain fixed — are 
therefore able to remove, and =~ well out to sea, a it 
that may have been left by the flood-tide, and at int same time 
Having so far endeavoured to bring forward general principles, 
itis now proposed to see how they can be applied in dealing with our 
rivers ; and in doing so, it must be remembered (to quote the words 
of Sir John Hawkshaw) “that there is nothing more certain than 
' that each one must be dealt with according to its own special 
régime.’ 
At the entrance to the Hunter River, Newcastle, there is a 
remarkable i of a natural breakwater, illustrating the 
advantage of protection works (natural or artifici tending into 
deep wate to the plan of the river taken in 1816, 
extending from the mainland to Nobby’s Head. But as this reef 
only afforded peskial shelter, we find the waves struggling with the 
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