152 iMr. Mallet on the Physical Conditions 



referring back to the same diagram, it will be seen that the great planes of 

 cross fracture usually turn out and meet the exterior of the gun, just at the 

 re-entering angle of the trunnion with the body of the gun. A gun, like every 

 other body that fails under strain, must fail in the weakest place ; we have 

 shown in what places they do fail, and we have shown a priori what must be 

 the weakest places (for a given material and mass), and we have found that 

 the places of fracture and positions of these " planes of weakness" most remark- 

 ably coincide. The conclusion, therefore, seems inevitable, that however 

 incapable the unaided eye may be to discern any difference in the crystalline 

 arrangement of one part of the gun more than of another, such planes of weak- 

 ness do exist, in the positions, and from the causes here pointed out. 



22. The external forms of cannon have been greatly modified and simplified 

 in modern times, from the complex and highly ornamented (?) forms of remoter 

 periods ; but eveu still, in the plainest forms of guns, such as Sir William Con- 

 greve's and Monk's patterns, &c., mouldings, astragals, reinforces, &c. &c., are 

 still adliered to, and from the unwillingness to give up altogether antiquated 

 forms, originally adopted and continued in ignorance, we have the folly still to 

 cling to making numerous and useless sharp angles and corners, and sudden 

 changes of form and of dimension on the exterior of all our ordnance, and so 

 prolong in the most needless way one cause of their weakness. That gun, how- 

 ever plain externally, will look best to the really educated eye, that most fully 

 conforms to the laws upon which its perfection as an instrument depends. 



5. — Physical Conditions induced in Moulding and Casting. 



23. Some remarks must now be added as to the effects upon the strength of 

 guns, which circumstances brought into play in the processes of moulding and 

 casting them exercise, extraneous to those which we have already treated of, 

 as respects the conditions of aggregation of the crystals of the metal. 



It is not my intention to go at any length into questions referring properly 

 to the iron-founder's art ; to practical methods, better or worse ; or to the details 

 upon which sound or defective castings depend. These, though most impor- 

 tant, do not find a fitting place here. But I purpose to consider : — 



