PROCEEDINGS OP THE POLYTECHNIC ASSOCIATION. 543 



inches from the inside, or *l inches from the axis, can only exert a force 

 25-49, or little more than half as much; 3 inches further, 10 inches from the 

 axis, the force exerted diminishes to 25-100, or but a quarter of that exerted 

 by the inside; and if the gun be 12 inches thick, the outside, which is 17 

 inches from the axis, can exert but 25-289, or about 1-12 as much power as 

 the inside. Of course, casting the gun still ihicker would add but very 

 little to its strength; we cannot, therefore, be astonished that it has been 

 found in practice that cylinders for hydraulic presses with a thickness equal 

 to about half the diameter of the piston are very nearly as strong as if ten 

 times as thick." 



14. " In 1855, Pr. llart, of Trinity College, Dublin, investigated the prob- 

 lem. His calculations (see note W., p. 259 of Mr. R. Mallet's work on the 

 Construction of Artillery) give the greater strength to the inner parts, but 

 still less to the outer, than those of Professor Barlow. Both these gentle- 

 men, as well as Gen. Morin and Dr. Robinson, the astronomer, who have 

 also studied the question, agree that no possible thickness can pnable a 

 cylinder fo bear a pressure from within greater on each square inch than 

 the tensile strength of a square inch bar of the material; that is to say, if 

 the tensile strength of cast iron be six tons per inch, a cylinder of that 

 metal, however thick, cannot bear a pressure from within of six tons per 

 inch." Hence the necessity of shrinking bands one upon another each with 

 a certain pressure upon what it encloses to get a gun into the state of so- 

 called "initial tension" necessary to its sustaining the greatest pressure 

 from within possible to a given thickness of wall, which as applied to a 

 hydraulic press cylinder is correct, but for a gun it is a fallacy; the effect 

 of the expansion of the interior by the heat of the powder not being taken 

 into the account. 



THE REQUIREMENTS OF ACTUAL SERVICE. 



15. Thus, solid or hollow-cast guns are useless for actual service and 

 against armored ships, which is now the means of defense to which our 

 means of offense must be adapted; and no proper preparation is made .for 

 such a contingency. The Navy Bureau design adopting the Rodman system 

 of casting hollow and cooling from the center, thereby admitting their want 

 of confidence in the Dahlgren plan. Yet this will increase their weakness 

 against the expansive force of heat engendered in battle firing. For it has 

 been seen that the Dahlgren 11-inch gun is not capable of enduring heavier 

 charges than 15 pounds of powder and a hollow shot, and that it is only 

 safe under slow firing, principall}' because of its porosity inside, whereas 

 if it-is cast hollow as in the Rodman plan, and cooled from the center, al- 

 though it will stand occasional heavy charges better than the other, and 

 have more enduring surface to the bore, it must burst when fired rapidly, 

 even with small charges, thereby rendering it entirely inefficient for actual 

 service. For the inevitable effect of the system is to produce an initial 

 tension to the verge of rupturing, rendering them certain to explode by the 

 force of the metal expanding when quickly heated from the interior; in fact 

 one of the Rodman guns burst itself on cooling in the foundry at Pittsburg 

 before it was taken out of the pit. 



IG. Of the classes of guiis that have, up to this date, been fabricated, 

 those of cast-iron are of three kinds: the Dahlgren gun, cast solid, the 



