466 SCIENTIFIC PAPERS OF SIR WM. SIEMENS. 



with his friend Mr. Walker that the same result would be obtained 

 by fluid compression. 



There was one more point which he might allude to. Mr. Parker 

 had shown on a table that the best results, as regards wear and 

 tear, were produced from a comparatively mild steel. He thought 

 it should be borne in mind that for all working parts of machinery 

 the ultimate strain which the metal had to endure did not exceed 

 two or three, or at most four tons to the square inch, for it was 

 inconceivable for a shaft, while it did its regular work, to be 

 strained in any part above four tons to the square inch. If the 

 steel was of the mildest description, the limit of elasticity would 

 be about fifteen tons. There was no object to be gained in raising 

 that limit up to twenty tons, and the ultimate strength to forty 

 tons, if they lost thereby that most valuable of all qualities in all 

 metals, that of ductility, or yielding before fracture took place. 

 All moving parts of machinery, such as shafts and cranks, he 

 thought should be made of steel of the mildest description. 



