554 TRANSACTIONS OP THE AMERICAN INSTITUTE. 



it, having- the effect to contract or squeeze it into smaller dimensions, both 

 longitudinally and radially. In this manner the state of "initial tension" 

 is attained in the g'un, which makes it capable of resisting a greater pres- 

 sure from within, having a tendency to rupture the wall or enlarge the 

 bore of the gun ; the necessity for which tension has been beautifully 

 described and illustrated by Professor Treadwell, of Harvard, and by Cap- 

 tain Blakely, R. A., England. Although the longitudinal tension might He 

 considered advantageous in assisting to resist the lengthwise pressure of 

 the powder against the bottom of the bore or chamber of the gun, there is 

 another rupturing force to be provided for, viz : the unequal heating when 

 it is lired ; for then this force has the tendency that cannot bo resisted by 

 an}'- amount of strength in the gun to increase both the longitudinal and 

 radial extension, for the part even slightly heated becomes both longer and 

 larger in diameter. 



SPONTANEOUS EXPLOSION LOSS TO THE FOUNDER. 



79. The Rodman gun, as cast at the great P^ort Pitt foundry, is strained 

 in this manner to the extreme limit of the elasticity of the outside metal, 

 and to the extreme limit of compressibility of the inside metal, which is 

 proved by the fact that one of them was ruptured at the abovenamed foun- 

 dry, from end to end, before it was removed from the mould in which it 

 was cast ; and the fissure, occurring through a wall of iron sixteen inches 

 thickness and about sixteen feet in length, remained in the close contact 

 along the bore, while it was gaping along the outside wide enough for the 

 insertion of the fingers. Like St. Rupert's drops, these guns are liable to 

 fly into pieces on slight occasions. How wonderful it is that any of them 

 endure firing at all. 



LOSS TO THE NATION, 



80. If a tightly-fitting tompion happens to be placed in each of the large 

 number of Rodman guns mounted in the forts of New York harbor, when' 

 for some time the weather has been warm, and then changes suddenly to 

 extreme cold, it is quite possible a number of them may be found bursted. 

 The tompion would protect the surface of the bore from the change of tem- 

 perature, which would dangerously contract the outside upon the inner 

 metal. We have never noticed whether these guns are furnished with 

 tompion or not. If they are, we would advise that they be not used. 



81. Guns with " initial tension" can withstand, more p?-e8.SMre than with- 

 out. Coihsequently, a single very heavy charge can be fired 'from these 

 guns without bursting them, while rapid tiring, such as practiced in battle, 

 will burst them even with small charges. When these guns are proved, 

 as well as in target practice, they are fired slowly. If we ever have to 

 resist the entrance to our harbors of a fleet of iron-clads, eacli gun will 

 have to be fired rapidly. What if they burst at that critical time ? No 

 fort or works on which these guns are mounted has ever been attacked. 

 They have never been subjected to the test of actual battle, yet hundreds 

 of them are being mounted on the works at the entrances to our most 

 important sea-port, and we rest in fancied security because they look 

 formidable. 



82. Of those mounted in our turret-ships, we believe eight were in the 



