368 
rOPULAE SCIENCE EEVIEW. 
what it is possible to accomplish. For whether we shall ever 
see used the thickest plates unbacked or not, it is certain that 
thick plates, will be required as face plates for all the composite 
targets, to take the main work out of the shot before it gets at 
the rear plates, if iron be put upon iron, or into the wood, or 
compound backing, whichever may be employed. Nothing 
has stopped a shot so effectually, or within less distance, or less 
time, than a solid plate of rolled iron ; and nothing will stop it 
quicker, or with less penetration, unless it be hardened steel, or 
chilled iron, both rigid materials, and consequently brittle. 
Experiments have all, so far, shown hard steel, chilled and cast 
iron, alike to be untrustworthy, and unsuitable materials. 
They are shattered by the shot like looking-glasses. Now a 
starred plate is a greatly damaged one ; every fresh blow, even 
if the shot does not penetrate, opens by vibration and buckle 
the fissures, and continues their courses. Portions are thus 
flaked away, and a ruin full of vulnerable spaces is presented to 
the enemy’s view. The inability to endure this starring is the 
fatal defect of the plank system, and one which is insuperable 
in it. Suppose an ordinary 8-inch plate, 4 feet 6 inches wide, 
to be pierced and starred by a 9-inch shot. There would be, 
as a general rule (for which there is an explicable reason), four 
rays or fissures passing away at right angles to each other from 
the shot-hole. These fissures are very seldom less in length 
than the diameter of the shot. Thus, with a 9-inch shot, from 
one end of one fissure to the termination of the opposite one 
would be 27 inches, on each side of which there would remain 
13'^ inches of the plate uninjured, and quite sufficient to hold 
the whole plate together. But if the same fissure were made — 
as the same shot would produce — in a plank of the same thick- 
ness, but only 16 J inches broad, the plank would be cracked 
completely asunder. If the plank were made of more tensile 
iron it would be only very slightly better off, for the more the 
plank bent the more the back of the plank would be torn apart, 
and ultimately a complete split would happen from a totally 
different cause. In the plate-up on-plate system, in which 
thick laminae of 5 or 6 inches of iron are put direct upon each 
other, the vibration and shearing action is immense, and the 
bolt heads and nuts are sure to fly unless extraordinary pre- 
cautions are taken. Moreover, this system is open to the very 
serious mechanical defect of presenting very weak spots at the 
cross or intersecting joints, when the plates of the difl*erent 
laminae are placed alternately horizontally and vertically. 
Thus in the Gribraltar shield all the plates were of the same 
dimensions, the face plates being laid horizontally and the rear 
ones vertically. The joints therefore intersected, and a shell 
fired at this point went clean through, clearing out a great 
