286 W. A. Norton— Experiments on Wood, Iron, and Steel bars. 



is greater if the stress is reached by a series of increasing 

 weights than if the full stress is directly applied. 



4. When the same strain is repeated on the same bar, after 

 a short interval of time, the set first obtained is not augmented, 

 unless the load applied exceeds a certain amount, varying 

 with the material and dimensions of the bar. With loads 

 greater than this limit each repetition of the load augments 

 the total set. The amount of the increase varies with the 

 interval of time since the previous application of the load 

 and the number of previous applications. 



5. The set, or residual depression of the middle of the bar, 

 experiences marked variations as the interval of time subse- 

 quent to the removal of the stress increases. When the imme- 

 diate set is Jess than about 00005 in. it passes off in a few min- 

 utes (10 m. or less). When it is greater than this it habitually 

 varies as follows : it invariably decreases for a short interval of 

 time, and then ordinarily increases for a longer interval, with 

 moderate fluctuations. The period of decrease varies from 

 about 5 m. to 20 m. ; and is the longer in those instances in 

 which the stress is prolonged. The subsequent increased set, 

 or augmented depression of the line of the bar, may attain in 

 less than an hour to an amount even greater than the set 

 observed immediately after the stress is withdrawn. In some 

 of the experiments the depression increased until it came to be 

 about double that first observed. The proportionate increase 

 of set is usually, however, much less than this. This increase 

 of set is eventually succeeded by another decrease. These re- 

 markable fluctuations observed in the line of the bar were more 

 conspicuous in the experiments with white pine, than in those 

 with iron and steel. The difference was, however, only in de- 

 gree. Under similar conditions the general character of the 

 fluctuations was the same whichever material was used. The 

 fluctuations observed with the bars of iron and steel, as well as 

 with the wooden bar, far exceeded any errors to which the 

 observations were liable. They were also much too slow, and 

 too prolonged, to be regarded as simple vibrations of the bar, 

 consequent on the removal of the downward pressure. 



6. Abnormal variations from the general law of variation of 

 the set just noticed, may occur under especial circumstances. 

 Such deviations were observed after the bar had been subjected 

 to repeated strains from day to day. Under these circum- 

 stances the bar may be in such an abnormal condition that the 

 set observed immediately after the stress is withdrawn may 

 pass off rapidly, and the line of the bar may even rise consid- 

 erably above the position held when the stress was applied— 

 though not above its original line some days previously, before 

 any strain was applied. 



