W. A. Nor ion — Experiments on Wood, Iron, and /Steel bars. 285 



machine is given in the Proceedings of the American Associa- 

 tion for the Advancement of Science, Eighteenth Meeting, 

 Aug., 1869, (p. 48). The depressions of the middle of the bar 

 experimented on, — while under a transverse stress, or remain- 

 ing after the stress has been withdrawn — are measured by it 

 to within tot? o o of an inch. The experiments on set have been 

 fully discussed in two papers read before the National Acad- 

 emy of Sciences, Washington, (April, 1874 and April, 1875). 

 The first paper set forth the results of the experiments on bars 

 of wood, and contained a detailed account of the course of ex- 

 periments instituted for the purpose of detecting instrumental 

 errors, and of the precautions taken to reduce the incidental 

 errors, from variations of temperature and other causes, to a 

 minimum. The second paper discussed the experiments on the 

 set of bars of wrought iron and steel ; which gave results gen- 

 erally similar, under corresponding circumstances, to those ob- 

 tained with wood. I propose, in the present communication, to 

 give a succinct statement of the general conclusions that follow 

 from the whole discussion. 



The experimental investigatiou'was prosecuted under three 

 general heads : 



I Sets from momentary strains. 



II. Sets from prolonged strains, 



III. Duration of set ; and variation of set with interval of 

 time elapsed after the withdrawal of the stress. 



Each of these embraced several special topics of inquiry. 

 The bars used in most of the experiments consisted of one of 

 white pine, 3 in. by 3 in and 4 ft. long ; another of wrought 

 iron, I in. wide, 1 i'n. deep, and 4 ft. long ; and a third of steel 

 of the same dimensions. The discussion of the entire series of 

 experiments has brought out the following results, as alike 

 applicable to bars of wrought iron, steel, and white pine. 



1. The immediate set, — that is, the residual deflection which 

 obtains immediately after the transverse stress is withdrawn,— 

 increases in nearly the same proportion as the stress applied ; 

 until this exceeds a certain amount, beyond which the set in- 

 creases according to a more rapid law than that of propor- 

 tionality to the strains. It is to be understood here that the 

 it considerable intervals of time, 

 [igments with the duration of the 

 stress, up to a certain interval of tima In the experiments 

 with white pine, the duration of strain which gave the maxi- 

 mum immediate set, varied, with the strain, from ten minutes 

 to one hour. The immediate set resulting from a prolonged 

 strain, was found to be from five to nine times as great as that 

 which succeeded a momentary strain. 



3. The residual depression below the original line of the bar, 

 ^- Jour. Sci.-Third Series, Vol. XI, No. 64.-Ahkil, 18T6. 



