involved in the Construction of Artillery. 255 



furnace ; but, however great these may be, the limit is at length reached by all ; 

 and with our existing tools in Great Britain is probably reached in every case, 

 at a diameter (of a cylindrical mass) of about 4 feet, and about 20 feet in 

 length. 



216. To the unpractised, though perhaps scientific observer, who looks at one 

 of those ponderous masses withdrawn from the furnace, glowing like a sun, and 

 observes the apparentlylittle effect that the thundering blows dealt by the steam- 

 hammer produce upon it, it always seems, that nothing more is demanded than 

 great increase of weight and length of stroke, or increased power in the hammer. 

 This, however, is a mistake : good forging, in heavy masses, depends not so much 

 upon the force of the blow, as upon its exact direction, and its application at the 

 precise moment when the welding metal is fit to receive it. The only effect of 

 great increase in the power, and especially in the velocity of the blow, is to shatter 

 and dislocate the internal or adjacent portions of the mass, which are at or about 

 a low (cherry red) heat, at which temperature the best wrought-iron appears 

 to lose much of its plasticity of heat, and be comparatively crumbly and brittle. 

 In fact, with existing hammers of 5 tons weight, and 6 foot blow, this effect is 

 very frequently actually produced. 



217. On masses of very large diameter, the effect of the heavy blow of the 

 steam-hammer is frequently to produce a singular form of internal hoUowness 

 and unsoundness, at or near the centre of the mass, where perfect soundness may 

 have existed in an early stage of the forging. The shock of each blow received 

 at the surface, and the reaction to which is the inertia of the more or less softened 

 mass in an exact opposite direction, gradually condenses the iron towards the 

 circumference, by drawing it away from the centre, where large cracks open with 

 rough torn opposing surfaces, and form irregular cavities. Sudden changes of 

 dimension, as when large projecting " collars" are forged down to a " shoulder," 

 produce the like effect from like causes. The mass is generally unsound 

 towards the centre within the larger part of the mass. The efiect may be 

 illustrated, and is of the same sort in fact, as though a barrel nearly full of 

 bullets were slowly turned on its axis and heavily struck at successive points 

 all round its exterior ; the bullets, at each blow, would all tend to jerk towards 

 the point struck, with energy proportioned to their nearness to the blow, and, 

 if by any means kept in the positions respectively assumed after each blow, 



2 L 2 



