MEMOIR OF THE LATE PROF. E. HODGKINSON, F.R.S. 189 



If e= elongation of a bar of cast iron one incli square 

 and (/) inches long by a weight Wy 



€ €* 



then 2^=13934040^ — 2907432000-^. 



If <;?= compression of a bar of cast iron one inch square 

 and (/) inches long by a weight w, 



then t«;= 12931560-^ — 522979200-^. 



These formulae were derived from the mean results of 

 four different kinds of cast iron. 



The mean tensile strength was found to be 157 n lbs. 

 per square inch^ and the ultimate extension was i-6ooth of 

 the length of the bar. 



With respect to wrought iron^ the extensions and com- 

 pressions were found to be nearly proportional to the pres- 

 sures producing them. 



The extension is proportional to the pressure up to about 

 12 tons per square inch, after this the pressure is not pro- 

 portional to the extension. The weight necessary to elon- 

 gate a bar of wrought iron to double its length is 27,69 1,200 

 lbs., which is usually called the modulus of elasticity. 

 One striking and important fact was elicited by these ex- 

 perimental researches, viz. cast-iron bars are decreased in 

 length double as much as wrought-iron bars by the same 

 pressure ; but wrought-iron bars sink to any degree with 

 little more than 1 2 tons pressure per square inch of section, 

 while cast iron-bars require three times the pressure to 

 produce the same effect. It appears also that the tensile 

 force of cast iron depends but little upon the form of the 

 section, except so far as the form contributes to the better 

 consolidation of the casting when in a fluid state. 



The above results were obtained for the Commissioners 

 by the individual labours of Mr. Hodgkinson himself, who 

 alone is responsible for their accuracy, usefulness, and 

 general adaptation to promote the ends of physical and 



