PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 387 



American Institute Polytechnic Association, ] 

 October 30, 1862. ) 

 Dr. R. P. Stevens in the chair. 



Cast Iron Forts. 



Mr J. Wyatt Reid. — Mr. Chairman, I liave here some models of cast iron 

 blocks which I have designed for the walls of forts. You see that the sur- 

 face is cast with alternate depressions and projections, so that the blocks 

 lock into each other; thus binding the whole wall together in such manner 

 that, if one block is broken out, the wall will not fall down. Indeed, 100 

 feet of the wall may be undermined without bringing it down. Projectiles 

 are now made of such weight, and propelled with such velocitj^ that, as we 

 have repeatedly seen, granite walls are soon drilled and crumbled to 

 pieces, and, unless some better material is brought into use, the use of 

 land fortifications will have to be abandoned. It has been proposed to 

 face the walls of forts with wrought iron, but we all know that as wrought 

 iron rusts on the surface, a scale of the oxyd falls oft", exposing another 

 portion of the metal to the action of the air, and thus the sheet is rapidly 

 worn away; while the scale of rust on cast iron adheres and forms a pro- 

 tecting coat which preserves the remainder of the mass from rust. These 

 blocks may be built up in front of the walls of our present forts, and they 

 may be made of whatever thickness is found necessary to resist the shot 

 in use, and then if more powerful missiles should be introduced, the walls 

 may be increased in thickness by raising an additional layer of blocks. 

 Cast iron will resist a crushing pressure of 100,000 pounds to the inch, 

 granite about 11,000, and brick, 4,000 or 5,000. 



Mr. J. K. Fisher, alluding to the question raised, as to the velocity of 

 shot, agreed to the proposition disputed by some, but now pretty well 

 established, that the force of a moving mass for desti'uctive effect, is as the 

 weight multiplied by the square of the velocity. He said the Horsefall 

 gun, in Great Britain, fired thirteen inch shot through the target repre- 

 senting the plating of the Warrior. Some iron acts very differently from 

 others. At Shoeburyness, in England, recently, one shot half buried 

 itself in one target without cracking the iron. 



The Secretary. — The range of rifled projectiles is greater than that of 

 round balls, but the initial velocity is not as great. The greater range 

 with a lower initial velocity is due to the greater momentum in proportion 

 to the resistance of the air, owing to the elongated form of the projectile. 



Mr. Fisher. — If these blocks are cast of several tons weight each, as 

 proposed, the difiSculty of breaking them will be very great. I recollect 

 seeing the efforts in progress at the Novelty Works to break a mass of 

 cast iron, which by some oversight or accident became chilled in the furnace. 

 After trying several more rapid plans, such as dropping weights upon the 

 mass from a great height, some very slow process was resorted to — I do 

 not know what. 



The Secretary. — It was drilled in lines and split to pieces with chisels. 



Mr. Fisher. — It was a mass of about thirty tons, but it did not look very 

 large — some six feet in diameter, I should think. 



The Secretary. — Five feet. 



