]
For these reasons, and in order to increase the power of resistance of a
cylinder, it is necessary to obtain on the inner layer a state of
initial compression approaching as nearly as possible to the elastic
limit of the metal. This proposition is in reality no novelty, since it
forms the basis of the theory of hooped guns, by means of which the
useful initial stresses which should be imparted to the metal throughout
the gun can be calculated, and the extent to which the gun is thereby
strengthened determined. The stresses which arise in a hollow cylinder
when it is formed of several layers forced on one upon another, with a
definite amount of shrinkage, we call the stress of built-up cylinders,
in order to distinguish them from natural stresses developed in
homogeneous masses, and which vary in character according to the
conditions of treatment which the metal has undergone. If we conceive a
hollow cylinder made up of a great number of very thin layers--for
instance, of wire wound on with a definite tension--in which case the
inner layer would represent the bore of the gun, then the distribution
of the internal stresses and their magnitude would very nearly approach
the ideally perfect useful stresses which should exist in a homogeneous
cylinder; but in hollow cylinders built up of two, three, and four
layers of great thickness, there would be a considerable deviation from
the conditions which should be aimed at.
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