436 



given to determine the position of the neutral axis, from which it is 

 found to be at the centre of gravity of the section, as in cast iron ; 

 so that the action is the same in both materials, except as to the 

 amount of the extensions and compressions with a given strain ; and 

 the formulae given for cast iron will also apply to wrought iron. As 

 wrought iron yields by bending and not by fracture, the relative value 

 of f and (f) are not so easily ascertained ; moreover the ultimate com- 

 pressive strain which wrought iron can sustain is little more than 

 half its ultimate tensile strength ; nevertheless the force required to 

 overcome the elasticity of the material is nearly the same, whether 

 applied as a compressive or tensile strain ; the difference being, that 

 the force which overcomes elasticity when applied as a compressive 

 strain leads to the destruction or distortion of the material, while, 

 in the case of the tensile strain, the elasticity may be overcome long 

 before the material yields by absolute rupture. 



A statement is given of the results of experiments made by Pro- 

 fessor Barlow, in 1837, to show the weights which overcome the 

 elasticity of the metal when applied transversely as compared with 

 the weight necessary to produce the same result when applied by 

 direct tension, and from these it is concluded that the resistance of 

 flexure in wrought iron, considered as a force acting evenly over the 

 surface, is nearly equal to one-half of the tensile resistance. 



In an Appendix to this paper, by Professor Barlow (read at the 

 following meeting), the preceding principles are applied to beams 

 and rafters of non-symmetrical section. 



With this view, the case of the double-flanged girder with un- 

 equal flanges is selected and discussed, and formulae deduced, which 

 are then tested by comparison with the results of experiments by 

 Prof. Hodgkinson, published in the ' Manchester Memoirs ; ' a selec- 

 jion being made of those in which the girders differed most from 

 each other in section, dimensions, and bearing-distance. The chief 

 particulars of these experiments are given, with diagrams showing 

 the forms of sections, and the values as obtained from the formulae 

 are stated. The value *of the direct tensile strength of cast iron 

 thus derived, falls between the limits of 1400 and 1/00. 



In the Reports of the Commissioners of Inquiry into the "Appli- 

 cation of Iron to Railway Structures," are given the results of about 

 fifty experiments on the direct tensile resistance of one-inch square 



