364 
POPULAR SCIENCE REVIEW. 
cohesion and resistance over a very restricted area as shall enable 
that restricted space to receive and endure the concentration of 
the assaulting fire from the enemy’s most powerful guns, and 
that so effectually that no harm shall come to the gun or to 
the gunners. We must do this with the least possible amount 
of material, the least possible depth of structure, and with the 
greatest economy of expense. A good shield sufficient to resist 
such guns as our own artillery could not be produced and fitted 
under lOOOL ; and there are not less than 300 imperatively called 
for at this moment for only a few of our most important works. 
It is evident, therefore, by the problem to be solved, as well as by 
the circumstances under which it must ever be regarded, that 
the utmost skill, care, and efficiency of purposeful design is 
absolutely essential. 
There are four main systems of shields or targets at this time 
prominently before the world : 1. Solid plates, represented at 
the late Shoeburyness trials by the 15-inch rolled plate of Sir 
John Brown & Co., of Sheffield, and the 15-inch hammered 
plate of the Thames Iron Ship-building Company; 2. The 
laminated, or pla,te-upon-plate system, and represented by the 
(xibraltar shield ; 3. The plank system, represented by the 
Plymouth casemate ; 4. The compound backing system, repre- 
sented mainly in one aspect by the Chalmers’ target, and in its 
other, and so far as yet proved the best form, represented by 
the Mill wall shield, designed and constructed by Mr. John 
Hughes. 
In the first place we must thoroughly understand what we 
have to provide against in making a shield. We have to keep 
out shot and shell of weights varying from 115 lbs. to 600 lbs. (the 
largest proportion of our own rifled guns being 250-pounders), all 
being thrown with terrific energy. These missiles, as used in the 
British service, are made of chilled metal upon Major Pal User’s 
system, and are intensely hard and of dense rigid molecular 
constitution. The Palliser shot will split up into the most 
terrible langridge, which if it passes into the defensive works 
will carry slaughter all around. Dashed from the face of the 
target, the splinters fly back in every direction for hundreds of 
yards ; but the metal of the shot will not squash or yield in 
form, each fragment falls perfectly cold upon the ground with 
its edges sharp and angular, totally unlike the flattened and 
distorted portions of common iron or soft steel projectiles, which 
burn the fingers that attempt to handle them with the heat set 
up by the violent motion produced by the friction of the inter- 
sliding particles in the mass of metal affected by the force of 
the blow. The Palliser shells have, in consequence of the thick- 
ness required to be given to their walls, very small bursting 
charges, which appear to assist by their explosion at the moment 
