392 Mr. Mallet on the Physical Conditions 



Note S.— (Sect. 2(55.) 



Physical Constants of the Materials for Gun-founding. 



The Reports of Colonel Talcott and of Mr. Wade (" Ordnance Reports, United States 

 Army, 1856") indicate generally that the ultimate cohesion, both of cast-iron and of bronze, 

 rises with increase of density, although several apparent anomalies are introduced, that 

 would have disappeared, under more rigorous and distinct methods of putting the questions 

 to experiment. Many of the data of these Reports, nevertheless, are amongst the most 

 valuable and important that have yet appeared. 



In bronze gun-castings the extremes of density and tenacity were found to vary from 

 the gun-head's specific gravity 8-353, and with so low an ultimate cohesion as 26011 lbs. 

 per square inch, up to specific gravity 8'896, and cohesion 56,360 lbs. per square inch. 

 The density was found steadily to increase with increased head of fluid metal, varying in 

 the same gun, e.g., one of about 90 inches length (12-pounder), thus: — 



Top of gun, specific gravity, . . 8-523; cohesion, 23,108 lbs. per square inch. 

 Base of gun, ,, . . 8-775; ,, 36,672 „ 



In another gun the cohesion varies from 26,426 lbs. to 52,192 lbs. per square inch. At 

 about yjjths of the ultimate cohesion, bronze is stated to begin to stretch and permanently 

 lose form ; this estimate is probably much too high. 



It is certainly surprising to find, throughout these generally valuable and elaborate 

 Reports, the most perfect neglect of the all-important constant of extension in relation to 

 strain. Ultimate cohesion, the final force of rupture, is systematically, and in very nume- 

 rous examples, ascertained, but the amount o£ extension by less strains, prior to rupture, 

 is not only neglected, but even the value of ascertaining such a constant at all, appears 

 to be unknown; for the testing machine, figured and described in detail, though capable 

 of determining the ultimate resistance to tension, compression, and transverse strain, and 

 the resistance and angle of torsion, appears actually i«ca/)aWe of giving the amounts of ex- 

 tension, under various strains, with any pretension to accuracy; the longest specimen pos- 

 sible to be tested thus, being limited to under 1 foot long (Prof.Hodgkinson's experiments on 

 extension of wrought-iron were conducted on bars oi fifty feet in length), so that it would 

 really appear that the importance of the coefficients T^ and Ty remains as yet unrecognised 

 by the United States Artillery authorities ; and the same seems to be the case at Woolwich, 

 where the testing machine is a duplicate of the American one, and, I believe, imported 

 thence. Compression, in relation to load or strain, this machine appears to determine, 

 although certainly with immeasurably less accuracy, than in the methods employed by 



