MEMOIR OF EATON IIODGKINSON. 223 



years tlie wliole of his time and al)ilitics wore devoted to the subjcets of this 

 inquiry. The exertions, both physical and mental, wliich he made at this period 

 for the advancement of engineering science were so great as materially to 

 affect his health and prostrate his powers. Immediately after the ])ublication of 

 tlie commissioners' report in 1849, he sought the restoration of his exhausted 

 faculties by a tour on the continent of Europe. 



His labors for this commission are published in the report, and comprise 114 

 closely printed pages. The high importance of these labors may be, to some 

 extent, inferred from the circumstance of the commissioners pointing them out 

 for special notice, "Although we are aware that to point out the labors of indi- 

 vidual members of the commission would be impossible, and tliat it may appear 

 invidious to single out one for praise, we cannot resist the exju'ession of our 

 thanks to ]Mr. Ilodgkinson for the zeal and intelligence with which he has car- 

 ried out the remarkable series of experiments which are detailed in the appen- 

 dix A to this Report, and which constitute a large proportion of thosQ which 

 have been already described." (Seethe Commissioners' Report, page 15.) Such, 

 then, was the estimate of the labors of Mr. Hodgkinson by Lord Wrottesley, 

 Professor Willis, Colonel James, Mr. Ronnie, and Mr. Cubitt; and it has beeu 

 amply confirmed by the engineering experience of the last 13 ^"ears. 



The objects for which Mr. Hodgkinson sought in this inquiry were — 



1. The determination of the longitudinal extensions and compressions of long 

 bars of cast and wrought iron by weights varied by equal increments, up to that 

 producing fracture. 



2. The establishment of general formulae connecting the longitudinal exten- 

 sions, and compressions, and sets of cast iron with the forces producing them. 



3. To determine the deflection of horizontal bars produced by various trans- 

 verse pressures, and to conqiare the effects with those produced by impacts. 



4. To determine general formulae connecting the transverse pressure, the 

 deflection, and set remaining after the pressure was removed. 



If £ = elongation of a bar of cast iron one inch square and (?) inches long bj 

 a weight iv, 



then 10 = 13934040 i —2907432000?-. 



If f?= compression of a bar of cast iron one inch square and [1] inches long I f 

 a weight iv, 



then er= 12931560^—522979200 4- 

 I r 



These formulae were derived from the mean results of four different kindb of 

 cast iron. 



The mean tensile strength was found to be 15,711 })0unds per square inch, and 

 the ultimate extension was 1-GOOth of the length of the bar. 



With respect to wrought iron, the extensions and compressions were found to 

 be nearly proportional to the pressures producing them. 



The extension is proportional to the })ressiu-e up to about 12 tons per square 

 inch ; after this the pressure is not proportional to the extension. The weight 

 neces.sary to elongate a bar of wrought iron to double its length is 27,091,200 

 pounds, which is usually called the modulus of elasticity. One striking and 

 important fact was elicited by these experimental researches, viz., cast-iron bars 

 are decreased in length double as much as wrought-iron bars by the same pres- 

 sure ; but wrought-iron bars sink to any degree with little more than 12 tons' 

 pressure per square inch of section, while cast-iron bars require three times the 

 pressure to produce the same eflect. It appears, also, that the tensile force of 

 cast iron depends but little upon the form of the soctio:;, exce])t so far as the 

 fomi contributes to the better consolidati(jn of the casting when in a fluid state. 



The above results were obtained Hov the commissii>ners by the individual 



