fluctuated to the other extreme, but at this stage of evolution guns had become well differentiated into two classes, the musket and pistol being representatives of the one, while the portable cannon was a type of the other. Each was crude in comparison with the war machines of to-day, but efficient enough to make Napoleon the terror of Europe. This warrior's celebrated remark that "God is on the side of the heaviest artillery" was an indication of his view that the limit had not been reached, and that the art of cannon construction was enough developed to warrant the making of yet larger guns.
In the War of 1812 an American officer, Colonel Bomford, introduced a large cast-iron gun, intended specially for seacoast defense by firing bombshells at long range. Up to this time cannon had been made with little or no provision for the variation of stress in different parts of the gun due to the exploding powder. It was known that this stress must be greatest around the seat of the charge, but no experiments had been made to determine even roughly the rate of decrease, although methods were already in use for ascertaining the initial velocity of the projectile shot forth. Bomford bored a hole into the side of a cannon and screwed into this a pistol barrel, with a bullet inserted. A definite charge of powder being exploded in the cannon, the velocity of the pistol bullet gave a measure of the pressure at that point. A series of holes being made in succession from muzzle to breech, the corresponding velocities of the discharged bullets gave an indication of the relative strengths needed to resist explosion and the thickness of metal required. The form of gun was therefore modified to suit the stress, and greater strength in proportion to weight was thus secured. To this improved gun he gave the name of columbiad. This style of gun was soon adopted in Europe, and long continued to be a standard.
But there were inherent weaknesses due to the very fact of employing cast metal. Assume a mass of hot liquid iron poured into a mold to form a solid cylinder, the central part of which is to be afterward bored out. The exterior surface cools first and becomes a rigid solid, while the whole mass has contracted but little. Gradually the interior hardens and crystallizes, but normal contraction is prevented by the rigidity of the exterior shell. The condition of the mass is much like that of a Rupert's drop of glass, which breaks into fragments as soon as the outer shell is broken. The weakest part of the cylinder is the axial region, which is removed by being bored out; but still the weakest parts of the completed gun are its inner surface and breech, the very parts against which the greatest force of the exploding charge is exerted. With such a gun the limit of safety is exceedingly uncertain. The vibration due to discharge weakens the cast iron,
