164 Mr. Mallet on the Physical Conditions 



51. The enormous time required by a large casting for cooling, especially 

 if left to cool in the moidd, and hence " jacketted," with its badly conducting ma- 

 terial (clay and sand) is not generally known. The hydraulic press cylinders 

 for raising the Britannia Bridge tubes, which were about 12 feet long, and about 

 3^ feet diameter, and weighed in the mould, perhaps, 20 tons, were found red 

 hot at the expiration of seventy-two hours after having been cast, and only 

 became cold enough to handle ten days after being stripped from the loam, and 

 required " feeding" for more than six hours after having been poured. During 

 the greater part of this time, molecular changes were going on, increasing the 

 coarseness of the crystalline grain of the metal, and reducing its tenacity. It 

 would have been much better practice to have kept the exterior of " the loam " 

 wet, and thus induced cooling by evaporation, as soon as ever the " setting" 

 of the metal had rendered it safe to do so. 



52. The cooling must be uniform, so far as uniformity is possible. This is 

 impossible, strictly, in any casting ; the approach to it is most difficult, in heavy 

 solid castings, such as guns and mortars, and hence the great advantage that 

 would result from a return to the antique practice of casting them hollow upon 

 suitably made " cores," as admitting of internal cooling by artificial means, such 

 as a current of air, at the same time that the outside is cooling. It is under- 

 stood that the American Government so requires its guns cast, and cools them by 

 a current of water passed into the interior — a practice of very doubtful advan- 

 tage, as not under sufficient control to insure avoidance of an evil greater than 

 that it is proposed to remedy, namely, cooling the interior of the gun much 

 faster than the exterior. 



53 Unequal cooling, especially if very rapid, involves all the injury, that 

 violent internal wrenching and straining can do to strength, — strains of the very 

 same character as those under which it is part of the purpose of this paper to 

 show, that guns burst, and which often, in the every-day practice of the iron- 

 founder, result in actual fracture. 



54. Guns have long been cast in a vertical position, and with a certain 

 amount of " head" of metal above the topmost part of the gun itself: one object 

 gained by this (of great value) is to afibrd a gathering place for all scoria or 

 other foreign matter, an end that might be much more effectually accompUshed, 

 were the metal always run into the cavity of the mould by " gaits" leading to the 



