ON THE MECHANICAL PROPERTIES OF STEEL. 273 



3/C 



where d is the depth of the steel bar, d^ that of the iron bar, and p =3-5, 

 the ratio of their units of working strength. 



_ iiut as the areas of similar section are as the squares of their like dimen- 

 sions, 



Section iron bar 3-5t <Z^ 



Section steel bar = d- =3-5- = 2-3052. 



Now taking the cost of iron at £7 per ton, and that of steel at .£12, we 

 have for the relative cost of the two materials of the same strength. 



Iron 7x2-3052 16-1364 



Steel - 12 - ~12~ = ^ ■^^^"' 



that is, the cost of the iron would be about 1-1- times that of the steel. 



In the case of railway bars and such constructions, besides this saving in 

 the cost of material, it must be borne in mind that the steel rail would last 

 four times as long as the iron rail. 



Tensile Strain. — Table II. 



Taking the mean of the results of the experiments on thirty of the best 

 specimens, we find the mean tenacity per sqiiare inch =47-7 tons. 



Now if we take 25 tons per square inch as the tenacity of the best Eno'lish 

 hammered iron iu bars, it follows that the tenacity of these steel bars will 

 be about twice (1-91 time) that of the iron bai's. 



Economic use of the Material. 



For bars of equal strength, undergoing tensUe strain, the iron bar should 

 be about twice the section of the steel bar; now if the cost of steel be -£12 

 per ton, and that of iron .£7, then, for a ton of metal in each case, the com- 

 parative cost of bars of equal strength will be 



Iron bars 7x1-91 13-37 



Steel bars"^ l2 = -i2~ = l"ll-*5 



that is, the cost of the iron would be more than once and one-tenth that of 

 the steel ; in this case, therefore, the steel would be the more economical 

 metal. The saving per ton of material would be ,£1-37, or £1 7s. 4|f?. 



The work producing rupture in the different specimens is very variable, 

 owing probably, to some extent, to the errors arising from the determina- 

 tion of such exceedingly small elongations. This irregularity would have 

 been avoided if the specimens had been of greater length, so that the elonga- 

 tions might have been ascertained with greater accuracy. 



The greatest value (6403) of this work of elongation is given in expt. 14, 

 where the breaking strain of the specimen is below the average, being only 

 about 40 tons per square inch. 



The specimen (see expt. IS) which had the greatest tenacity, viz. about 

 60 tons per square inch, required only 670 units of work to produce rup- 

 ture ; this arises from the very small elongation, viz. -01, which the bar 

 sustained at the point of rupture. 



1867. V 



