THE PLYMOUTH BREAKWATER EORT. 
167 
we put the cost of the Plymouth fort, as it now stands, with 
the estimate of a comparative structure upon Mr. Hughes’s 
plan, we shall find that no saving has been effected by employ- 
ing bars instead of hollow stringers ; whilst the advantage of 
the latter would be very great in respect to defensive resistance. 
The accompanying woodcut will show the general structure 
of the walls of the Plymouth fort as they now stand. The 
front armour (a) is of rolled plates 5 inches thick ; behind this 
is a layer of shingle concrete 6 inches thick ; next a layer of 
SECTION OF IRON WALL, SHOWING SYSTEM OF STRUCTURE. 
a. Face armour ; * Concrete ; b. Vertical bars ; c. Horizontal bars ; t Iron concrete ; 
d. Wood plank, with iron struts at intervals ; x x x. Bolts, backing ; p. Perpendicular ; 
G. Glacis ; m m. Masonry foundation. 
vertical iron bars (6) 16 J inches wide and 5 inches thick ; then a 
similar layer of bars (c) laid horizontally ; behind this a plaster- 
ing of iron turnings and asphalte 1 inch thick. In the rear 
the wall is supported by vertical iron bars 12 inches deep and 
5 inches thick, the intervals (about 10 inches) being packed 
with wood planks set edgewise. A glance at the composition 
shows that there is not the slightest mechanical cohesion in it, 
but that the whole mass is simply kept together by the bolts. 
If these give way, the disruption of the wall seems inevitable. 
