256 
PROCEEDINGS OF THE VICTORIA INSTITUTE. 
parapet, on the landward side are alcoves, to give shelter to 
persons on the road level when the sea is -breaking over the 
parapet, as it very frequently does. The roadway is paved, and 
usually has one or more railway lines on it. 
I have so far tried to give a slight account of some pro- 
tective or refuge works. Turning now to harbours intended 
purely or mainly for commerce, those in England are mostly 
formed by the lower reaches of rivers, and their adaptation for 
trade consists in the construction of docks, ouays, shipping 
places for coal, should that be one of their exports, deepening 
if necessary the channel, and so on. 
Docks . — For a dock; the site having been selected, trenches 
are sunk on the lines of the enclosing walls and when excavated 
to the requisite depth the walls are built therein, usually of 
concrete blocks, backed with concrete in mass at the same 
time the excavation of the inferior of the dock proceeds. This 
is usually done by means of steam navvies, as they are called, 
which consist of a digging bucket capable of lifting about a 
cubic yard of material, on the arm of a special steam crane. 
A steam navvy with its 4 or 5 attendants will do the work of * 
gang of some 60 human navvies and at a considerably smaller 
cost. In constructing the Manchester ship canal, several dif- 
ferent kinds of diggers were used, including some very large 
ones with an endless chain of buckets, on' the principle of a 
dredger. The outer walls of a dock are sometimes constructed 
in the water on the same lines as sea-works. Docks have usually 
two entrances, side by side, one a tidal entrance, through which 
ships can enter only when the water outside is at the same 
level as that in the dock, and the gates of which are opened at 
about half flood tide, remaining open till the tide falls to this 
level again; the other a lock through which ships can be 
admitted at any time, by first raising or lowering the water in 
o t ie evel of that- outside, and then goin# through the 
opposite process till the water is the same level as that in the 
after "tt * rf - dock ! it Coble Dene on the Tyne, afterwards call 
Dock ’ \ nCe of . ^ales, who opened it, the “ Albert Ed« 
w de^, ^n1- \ tl , dal entrance SO feet wide and a lock 6U f 
eutVance f , ^ The dfi P th water on the sill of « 
P “ rii; ' ( mnfir f SOI of the lock is 30 feet. at high wa^ 
an aL of Acres' The dee P e " Thi * dock 1 
self-actin" ’ J 1 ? P rmCi pal export is coal, and for tl 
shipping °1 000 tor °* f ^PP, 111 ® Peaces are provided, capable 
PPmg 1,000 tons of coal per hour. For the import tra 
