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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 indi- 

 cation 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 1818 an American officer, Colonel Bomford, in- 

 troduced 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 varia- 

 tion 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 de- 

 termine 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 in- 

 serted. A definite charge of powder being exploded in the can- 

 non, the velocity of the pistol bullet gave a measure of the pres- 

 sure at that point. A series of holes being made in succession 

 from muzzle to breech, the corresponding velocities of the dis- 

 charged 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 im- 

 proved 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 nor- 

 mal 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 un- 

 certain. The vibration due to discharge weakens the cast iron. 



