THE ROYAL ARTILLERY INSTITUTION. 
209 
subject the bar, which has been elongated by the permanent exten¬ 
sion E G-, to a heavier pressure, then the total extensions may be 
represented by same line GK ,* and we see that the “ breaking strain/ 5 
shown by AL is greater than before. The “ total mechanical force 55 
to produce rupture will now be represented by the area AKL, which, 
however, can never exceed the mechanical force represented by ABL , 
or which is the same thing FGKL cannot exceed BDFE, so that the 
absolute strength of the metal is not increased by permanent extension 
beyond its original “ limit of elasticity/ 5 although its breaking strain 
and elasticity may be increased by the operation. 
We must then remember that increase in the limits of fracture (or 
breaking strain) and elasticity do not imply greater working strength in 
a given bar of metal. 
We do, however, gain very much, as we all know, by subjecting 
metals to the operations of rolling, hammering, &c., for we obtain 
greater elasticity and ultimate tenacity in smaller bulk by making the 
mass more homogenous; and here seems to be the weak point of the 
critique in question, for although steel might undoubtedly be further 
improved, the question lies between the present ringed Krupp guns 
and those of bronze steel as now made. 
The points of comparison pro and con , are as follows :— 
1 .—“ In endurance the bronze steel gun can only be compared with 
the ringed steel gun, since internally they possess the same strength, 
homogeneity and hardness ; while an elastic pressure is exerted prima¬ 
rily from without inwards, exceeding that exerted by the bursting 
effect of the powder. 55 
Remark on above .—“ The tension previous to firing, is in the bronze 
steel gun in exactly an opposite state to the tension in a Krupp ringed 
gun. Hence the two pieces admit of no comparison. 55 
The criticism seems scarcely justified as it stands, for the tension of 
most of the layers of the bronze-steel gun, certainly of all the outer 
ones, is of the same character to that of the steel rings, although 
primarily induced in a different manner. 
To give a simple example.—Suppose we take a roll of stiff paper, 
f-inch internal diameter, which requires some little force exerted from 
the interior, to expand it to an inch diameter, insert inside a pair of 
glove stretchers, and on the exterior place an india-rubber band just 
fitting over the roll of paper ; expand the latter from the interior, till 
its diameter is one inch, by means of the stretchers, and we see that the 
whole thickness of the band will be in a state of tension, representing 
that of the exterior layers in the bronze steel gun. 
The compression of the inner layer of compressible bronze steel cannot 
of course be simulated in this case. 
Again, take a roll of similar paper, say an inch and a quarter external 
diameter, place over it a thin india-rubber ring, (say |-inch internal 
diameter) of not sufficient strength to compress the paper roll more 
* If the metal were perfectly homogeneous, LK would correspond with BD; but as 
most metals are in a state of internal tension, which is nullified by pressure beyond their 
iimit of elasticity, the limit of fracture may frequently be thus increased. 
