464 THE SCIENTIFIC PAPERS OP 



ing, I am quite certain that the strain put upon it by expansion 

 would have had for its results simply a local elongation, but there 

 would have been accommodation ; whereas rigid material has no 

 accommodation, but must yield when, through local action at any 

 one point, the strain put upon it exceeds its absolute strength. 

 This, therefore, appears to me to furnish a very powerful argu- 

 ment in favour of a material of high ductility, and the fault I 

 would find with all these rules is, that the factor of ductility is 

 not introduced as part of the factor of safety. Now, Sir Joseph 

 Whitworth, who has given much attention to this subject, has laid 

 it down as a rule that the value of a material is composed of the 

 sum of its ultimate strength and of its yielding power under strain. 

 I believe that this is a very excellent rule. Therefore if the factor 

 of safety of 5 is none too much for a material that would break 

 when extended 10 per cent., if it could be made to extend 20 per 

 cent., the factor 5 ought to come down to the factor 4 ; and you 

 would get with that factor at least as much safety as you would 

 get with the factor 5, and the lower extensibility of the material. 

 This is a question which I hope will be very fully and carefully 

 discussed, and I trust that it will lead to a Eule in which both the 

 Board of Trade and Lloyd's surveyors concur, in order to prevent 

 the recurrence of such incidents as I have given you. 



In the discussion of Uw Paper 



"ON THE USE OF STEEL CASTINGS IN LIEU OF 

 IRON AND STEEL FORGINGS FOR SHIP AND 

 MARINE ENGINE CONSTRUCTION," by MR. WILLIAM 

 PARKER, Chief Engineer Surveyor of Lloyd's Register of 

 Shipping, 



SIR WILLIAM SIEMENS * said he thought the Institute should 

 accord their most hearty thanks to Mr. Parker for his very prac- 



* Excerpt Journal of tlie Iron and Steel Institute, 1883, pp. 89-90. 



