ARMOURED DEFENCES. 
323 
help comparison, the diagram shows also a shot in the act of piercing 
a single plate. 
It may be well here to mention that onr plate-npon-plate construction 
is, perhaps, more than any other, proof against what is called “ racking 
action”—that is, the effect of heavy shot striking at comparatively 
low velocities. 
The next trials on a large scale, which influenced our proceedings to 
an important extent, were those of 1868, at Shoeburyness. 
In these, a casemate, having a front of 22 ft. x 14 ft., representing Experi- 
a portion of the iron fort which was then in course of construction for “ e S emIte 
the position behind Plymouth Breakwater Fort, stood 37 rounds from gpresentj n g 
the 12-in. gun of 25-tons, charge 76 lbs. pellet powder, the 10-in. breakwater 
gun of 18 tons, charge 60 lbs. R.L.Gr., and the 15-in. Rodman gun of lort * 
19 tons, charge 100 lbs. American (equivalent to 83Jlbs. English) 
powder, at 200 yds. range ; and, although it was of course considerably 
damaged by this fire, it was, at the end of it, pronounced to be 
defensible. This trial led to some important additions being made to 
the front wall of the fort itself, as it went on. 
The roof also of this casemate was tested by the fire of 13-in. 
mortars, at 1000 yds., but they proved quite powerless against it. 
Only a few shell, however, struck it out of nearly 300 rounds. 
Adjoining this casemate was another, embodying several kinds of Cellular 
cellular construction in its iron front, with the object of comparing ca°semate. ted 
the resistance of moderately thick solid plates with that of thinner 
front plates supported by cellular compound backing; but, in no 
instance, did the latter construction prove itself superior to the other. 
This result was borne out by the trial of another shield in 1868. The 
support given to armour by massive piers of masonry and concrete, 
cased in thick boiler-plate, proved very satisfactory. 
It may be well here to notice briefly the matter of holding on Armour 
armour plates, on which so much of course depends, and I may begin bolts * 
by saying that of the innumerable contrivances for this object, nothing 
has been found equal to that of a simple screwed wrought-iron bolt 
fitted with nuts. 
The steps by which the present pattern of armour bolt for fortifica¬ 
tions has been arrived at may be thus described :— 
At first, a bolt with a deep v-shaped screw thread was used, but, in the 
early stages of our trials, a shallow round-cut thread was substituted for 
this with great advantage. Also a gradual and slight cone was adopted 
in lieu of the abrupt and spreading cone used at first in the heads of 
these bolts. 
Next, Sir W. PallisePs valuable suggestion that part of the shank 
or stem of an armour bolt should be reduced to the lesser diameter of 
the thread, led to an immense improvement, and this has been since 
extended to the whole length of the stem, leaving the thread a plus or 
raised thread, thus facilitating the extension of the material of the 
bolt throughout its whole length instead of locally in short lengths. 
Then Captain English proposed, first, the rounding of the bearing 
surface of a common hexagon nut, and, afterwards, the use of a 
spherical nut seated in a cup-shaped hole in the armour, or in a special 
