254 JMr. Mallet on the Physical Conditions 



sure for weeks, perliaps, to a temperature at which the exterior of the mass 

 gets changed more or less in chemical constitution, and at each welding the risk 

 of inclusion of more or less slag, cinder, or other foreign matter. (Note R.) 



At every additional piece thus laid on by welding, an additional doubt is 

 produced, as to whether or not the weld be sound throughout, — no examination 

 at the time can with certainty decide this. The mass, however, grows continually 

 in bulk and weight; the inertia of the hammer (large and powerful as this has 

 become through the intervention of the direct action of steam) becomes reduced 

 in relation to that of the mass in the same ratio ; the blow no longer acts with 

 uniformity upon the mass submitted to it, but is nearly confined in effect to the 

 immediate point struck. The mass, if very large, and especially if also long, can- 

 not be all maintained hot, between the portions at a welding heat and those 

 nearly cold, there are others at every temperature, and a large proportion at a 

 " low dull red," a heat at which all wrought-iron is more or less crumbly and 

 brittle. The jar and shattering vibration of every blow, as it thunders down 

 upon the huge piece, is transferred to the crystalline particles of these colder or 

 quite cold portions, and probably produces at length some considerable altera- 

 tion of molecular arrangement, in deterioration of strength, and often, before 

 completion, actually shakes the mass in two, at some point or other. 



215. At length the limit is found, when, with our present known modes of 

 working wrought-iron (even with the heaviest and best appliances), we can no 

 longer add to its size. The limit is reached, by the failure of power to heat the 

 mass, or the required part of it, to the welding heat. The time required for the 

 piece to remain in the furnace to effect this, continually increases as its bulk grows, 

 and with it, the sources through which heat is lost and dissipated ; but a certain 

 proportion of iron is burned away, or melted off from the surface at the part 

 requiring to be brought to welding heat, and from the adjacent portions at every 

 moment that it remains in the furnace, at last, as much in weight is burned off, 

 and lost at each welding, as equals the weight of the " slab" or mass laid on, and 

 the labour is then in vain ; the work, like that of the embroidery of Pene- 

 lope, becomes an endless task, and the limit has been reached, beyond which 

 the piece can be forged no bigger. 



The point at which this limit is reached, can be stretched a good deal by the 

 extreme skill of the operative forgeman, and the skilful construction of his 



