364 Mr. Mallet on the Physical Conditions 



at any point round the circumference is greatest ; but this, in a mortar 

 inclined about 45°, will be at the lines cut by a vertical plane passing through 

 the axis, — because the whole interior may be assumed heated equally all round ; 

 but, of the exterior circumference, the lower side, that opposite the vent, is 

 most cooled, because against this side of the warmed mortar the ascending 

 currents of air impinge, and most completely evect the heat from it; — while 

 at the opposite, or top side, the heat is least evected : the splitting tension is 

 greatest, therefore, in the plane through which these three mortars have been 

 split. Besides this, on the principles of molecular aggregation of castings, 

 previously explained, the mass ^the trunnions at either side produces a plane 

 of relative weakness towards th^case of the mortar, just where fracture has 

 occurred ; and lastly, the section through the vent is the weakest of any 

 one passing through the axis, that the mortar presents, because the least total 

 section of metal. 

 The excavating that has taken place at the bottom of the chambers is easily ac- 

 counted for. The metal — coarse, uneven, and open-grained throughout — was at this point, 

 from causes pointed out (sect. 45 of text), a spongy mass of scarcely coherent crystals, 

 with scales of uncombined graphite mixed with them and interposed; the latter in 

 the first instance, and the plumose crystals of cast-iron (iron and combined carbon) after- 

 wards, exposed to the intense heat and flame of the ignited powder, are themselves set on 

 fire, and gradually deflagrated ; and so, bit by bit, the irregular little cavern was burned out, 

 just where the central " soft spot" in the casting existed. It is a case precisely analogous 

 to the enlargement of vent and of chase near the seat of the shot, so commonly observed in 

 guns. 



The remedy for all these evils is not difficult, and was, in fact, pointed out by me in a 

 communication as to a new form of mortar, made to Government early in 1855, but put 

 aside on grounds that only proved that it had been set aside unconsidered, or the want of 

 information, on the part of the examining authorities, to enable them to judge such ques- 

 tions, which demand, not " artillery practice," but a clear and accurate knowledge of many, 

 and some not very obvious, physical and mechanical truths, and the power practically to 

 apply them. 1 recommended that mortars should be increased in length of chase ; the thick- 

 ness of metal reduced ; made perfectly uniform all round, and proportioned to the internal 

 pressure at every point, — i. e. tapering to the muzzle ; to abandon all trimnions (as weak- 

 ening the piece) ; and to receive the recoil directly from a flat breech, by an elastic, and 

 simple but peculiar bed ; and to cast all mortars hollow, on well- formed " cores," without 

 subsequently boring them out at all, — thus not only saving greatly in first cost, but avoid- 

 ing, as respects the molecular arrangement of the material, all the evils resulting from the 

 existing cumbrous and absurd forms, so that the form recommended would have approached. 



