PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 545 



powder (800 rounds with shell, and 200 with shot). The iron selected at 

 that foundry, and from which the last five experimental guns have been 

 made, was of the same quality, and in the same proportion as in the guns 

 last above referred to. 



24. In 1858, after the failure at the 169th fire of the West Point experi- 

 mental gun made from this iron, Mr. Parrott condemned it as being too 

 high for heavy guns. 



25. And again, " these facts to my mind are conclusive as to the fact 

 that we are at present far from possessing a practical knowledge of the 

 properties of cast-iron in its application to gun foundering; and it is too 

 much to expect of private enterprise to take up and prosecute so intricate 

 and expensive an inquiry." Thus we see how recently the author of the 

 system of casting hollow, acknowledges his ignorance of casting iron, and 

 I venture to say that neither Bureau knows more to-day. Colonel Rod- 

 man's plan of hollow casting provides well for the pressure of the powder 

 in a gun, but the gun that is in the best state to resist the pressure, viz: 

 by initial tension, is in the worst state to resist the unequal expansion from 

 the heat of firing; the same holds good in a wrought-iron or steel gun. 



FOR PRESSURE. 



20. Built up cylinders, i. e. with bands shrunk upon an inner tube of all 

 kinds are better able to withstand pressure than solid forgings or castings, 

 as has been her:tofore and will be hereinafter more fully explained; but 

 must be made of soft ductile iron that will stretch, to thus be disabled and 

 need repairing, or they will burst by unequal heating if made to be used as 

 guns. I 



EFFECT OF INITIAL STRAINS. 



27. The tensile strength of gun metals is usually only tested by simple 

 extension of length, one of the dimensions undergoing which extension 

 other dimensions are reduced. It is not known how the tensile strength 

 of a sample would be. affected if force was applied to extend all its dimen- 

 sions at the same time. As for instance, take a hollow cylinder of which 

 the area of cross-section is known and subject it to a pressure of liquid on 

 the interior, having only the tendency to enlarge its dimensions radially, 

 while the measured force of the instrument by which its tensile strength 

 was being measured, was applied to extend its length only. Or, better, 

 have the forces acting to enlarge the diameter pulling from the outside 

 instead oi pushing from the inside, it would undoubtedly be found that 

 while subjected to such additional forces the extension or rupture would be 

 accomplished with less force. For the attraction of cohesion acts with 

 greater energy as the faces of crystals or atoms are near or distant, in the 

 same ratio as the shadow of a screen upon a wall from a point of light in- 

 creases or diminishes in superficial area as the screen is moved toward or 

 from the light; hence the more dense the sample of cast-iron from rcmelt- 

 ing, or greater time in fusion, the greater the tensible strength. When the 

 initial tensions within the mass of the sample, from unequal cooling or 

 heating, do not affect the result, the extension of length would require 

 more force, because the faces of crystals are not all perpendicular to any 



[Am. Inst.] *I 



