Bessemer process for producing ingots of any size at about the cost of wrought-iron.  These and other makes of low-steel have endured extraordinary tests in the form of small guns and other structures subject to concussion and strain; and both the theory and all the evidence that we have promise its superiority for gun-metal.  But another element of resistance is required in guns with thick walls.  The explosion of the powder is so instantaneous that the exterior parts of the metal do not have time to act before the inner parts are strained beyond endurance.  In order to bring all parts of a great mass of metal into simultaneous tension, Blakely and others have hooped an inner tube with rings having a successively higher initial tension.  The inner tube is therefore under compression, and the outer ring under a considerable tension, when the gun is at rest, but all parts are strained simultaneously and alike when the gun is under pressure.  The Parrott and Whitworth cannon are constructed on this principle, and there has been some practice in winding tubes with square steel wire to secure the most uniform gradation of tension at the least cost.  There is some difficulty as yet in fastening the wire and giving the gun proper longitudinal strength.  Mr. Wiard, of New York, makes an ingenious argument to show that large cannon burst from the expansion of the inner part of the gun by the heat of frequent successive explosions.  In this he is sustained to some extent by Mr. Mallet, of Dublin.  The greater the enlargement of the inner layer of metal, the less valuable is the above principle of initial tension.  In fact, placing the inner part of the gun in initial tension and the outer part in compression would better resist the effect of internal heat.  But Mr. Wiard believes that the longitudinal expansion of the inner stratum of the gun is the principal source of strain.  A gun made of annular tubes meets this part of the difficulty; for, if the inner tube is excessively heated, it can elongate and slip a little within those surrounding it, without disturbing them.  In fact, the inner tube of the Armstrong gun is sometimes turned within the others by the inertia of the rifled projectile.  On the whole, then, hooping an inner steel tube with successively tighter steel rings, or, what is better, tubes, is the probable direction of improvement in heavy ordnance.  An inner tube of iron, cast hollow on Rodman’s plan, so as to avoid an inherent rupturing strain, and hooped with low-steel without welds, would be cheaper and very strong.  An obvious conclusion is, that perfect elasticity in the metal would successfully meet all the foregoing causes of rupture.