can stretch out beyond its elastic limit without breaking ; it thus con- 

 tinues to contribute to the strength of the whole. 



In general, forged metals will take a greater permanent deformation 

 than cast metals : they are more malleable. This is the case with steel. 

 We were very happy when they told us at Terni that they possessed 

 a press powerful enough to forge the two hemispheres of our cabin. 

 Over and above the superiority of the material itself, this process of 

 manufacture had other advantages. 



In a casting the upper part is apt to be of lesser quality. It is there 

 that one most often encounters flaws and pockets arising from gaseous 

 discharges. The raw material for each hemisphere formed an ingot 

 cast in a vertical position, 12-70 ft. high and of a mean diameter of 

 3-5 ft.: this ingot weighed 24 tons. One could thus sacrifice all the 

 upper part of the cylinder before beginning to forge the material in 

 order to utilize only the lower part, which would be of much better 

 quality. This part would then be flattened by means of a press, so as 

 to give it the shape of a disc. This disc would then be pressed into the 

 shape of a hemisphere. Thus each pocket would be flattened and would 

 finally form a thin vein which would run perpendicular to the radial 

 line of the sphere. The elementary theory of strength of materials 

 shows that in these conditions a small initial defect would hardly 

 decrease the strength of the sphere at all. 



AT THE FORGE 



Here we are in the immense forging shop at Terni. The door of one 

 of the greatest furnaces rises. It is so dazzling at first that nothing 

 can be seen inside. If a smoked glass is used, however, an incandescent 

 block can be distinguished. A giant tool, suspended from a travelling 

 crane, transports this radiant mass across the hall and places it upright 

 on the bed of the press. Now imagine this press exerting 12,000 tons, 

 probably the most powerful in Europe, if not in the world, while its 

 ram, actuated by three hydraulic cylinders, is lowered slowly: under 

 its pressure the block is compressed and broadens out. Upon contact 

 with the air, a layer of oxide is formed on the block of metal : less hot 

 than the interior of the ingot, it crumbles under the action of the press 

 and the dazzling steel appears before oxidizing once more. 



The work is not done in one operation. The block returns to the 

 furnace several times and stays there for several hours each time 

 before the operation is continued. At one time it is as broad as it is 



[84] 



