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MINUTES OF PROCEEDINGS OF 
Independently, however, of the increased strain, the grooving of the bore 
weakens the metal, especially if cast. 
Before passing on to consider the materials for ordnance, I must also 
remind you, that if a gun consists of one solid mass of metal bored out, the 
exterior portions take but a small share of the strain. I need not dwell 
on this point, which has been so fully explained by Mr Mallet, Captain 
Blakely, and others; and you are doubtless aware of the several proposals 
made and some of the methods adopted for equalizing the strains upon 
different portions of the metal of a piece of ordnance. 
Materials for Ordnance . 
The four materials generally used for ordnance are bronze, cast-iron, 
wrought-iron, and steel. In selecting a material for ordnance it must be 
remembered that it will not be subjected to a mere variable pressure, but 
that the force of the gas is percussive, resembling a blow rather than a 
pressure, and therefore that the tenacity of a metal is not always a criterion 
of its suitability for guns. The properties especially required in a metal for 
cannon are elasticity, toughness (tenacity combined with flexibility), and 
hardness; ductility will allow a greater margin of safety beyond the elastic 
limit, but the strain should, if possible, be confined within this limit.^ It 
is also important that a material capable of being manufactured with 
certainty be chosen, and that both the material and the method of con¬ 
struction be not too costly. 
Bronze is sufficiently tough, and can, with skill and care, be cast so as to 
be uniform in quality, but it is costly and too soft for the bores of rifled 
guns; it does not corrode like iron, and therefore requires no painting. 
Cast-iron possesses the necessary hardness, but is very brittle, and, even 
with the greatest care in casting, is of uncertain tenacity. The Americans 
have of late greatly improved the quality of the cast-iron ordnance by the 
selection of the most suitable ores, aud by taking means to render uniform 
the contraction of the different portions of the metal in cooling; never¬ 
theless, cast-iron cannot be used for rifled guns, unless they are fired with 
Very small charges. 
Many attempts have been made to strengthen cast-iron guns by shrinking 
Or driving wrought-iron or steel rings or tubes over the breech; the jacket 
thus applied, being in a state of tension, should take a fair share of the 
strain; and as the cast-iron will be compressed, it will be in a condition 
favourable to resistance to strain. Numerous experiments have, however, 
shewn that cast-iron guns strengthened on the outside will not stand the 
great strain caused by firing heavy elongated projectiles with even moderate 
charges and this result has been confirmed by the many failures of the Parrott 
rifled guns in America. A better plan is to place the wrought-iron or steel 
inside the cast-iron, as proposed by Major Palliser, the stronger metal being 
then in the position where it is the most required. 
* According to Barlow, no thickness of metal can permanently resist an internal strain greater 
than the tensile strength of the material. 
