304 Mr. Mallet oji the Physical Conditions 



The first particles of graphite removed, produce a slightly increased area of 

 passage at one place, which results in a greater velocity of the issuing flame 

 and gases at this place of greater area, (on the known principles of Pneuma- 

 dynamics) ; but the greater velocity here, produces again a greater proportionate 

 wear and abrasion ; hence, the moment the cylindrical figure of symmetry, is 

 once lost, the tendency to produce irregularity of section is constantly aggran- 

 dized up to a very wide limit, when, at a wr?/ large section of aperture, the wear 

 all round would again tend to return to uniformity, with a certain constant of 

 surface irregularity, dependent upon the degree of heterogeneity of the material. 



This stelliform cutting away of the cast-iron, at the vents of the Columbiads, 

 and other large guns, was found after 300 rounds (8-in. guns) to exceed an 

 inch in diameter from point to point, and after 600 rounds could not be 

 embraced by a circle of two inches diameter. In no single respect, is the minute 

 subdivision of the graphite, and uniformity of testui-e in the cast-iron for guns, 

 so impoi'tant as in this, and in none would the superiority of wrought-iron be 

 more manifest. 



309. The polish or burnishing of the lower surface of the bore, at the place 

 of the shot, proves that its rotation, by friction against the chase, does not com- 

 mence instantly upon its first movement, but that it slides for a short distance 

 before the frictional grasp of the ball against the bore is able to overcome the 

 inertia of the shot so as to produce rotation ; and this also indicates, that at 

 every grazing stroke, afterwards, of the shot against the side of the chase in its 

 progress towards the muzzle, the mutual friction must be that of rubbing or 

 sliding, and not that of rolling surfaces ; indeed of rubbing surfaces often, with 

 proper contrary motions, when the ball has acquired previous rotation. So 

 that hallotage, is not a wear merely by the grazing stroke of the ball at a very 

 small angle, condensing and pitting the substance of the gun ; but an actual 

 abrasion and degradation of its substance, due to rubbing friction at these 

 points of maximum pressure, carried beyond the limits of endurance of the 

 metals. 



310. These circumstances throw much light upon the great economy in 

 wear of the chase, produced by the use of the sabots or other solid wads. It is 

 obvious also from the consideration of all the conditions of wear here pointed 

 out, that any conclusions as to material for guns derived from tests of hardness 



