W. A. Horton— Experiments on Wood, Iran, and Steel bars. 287 



7. When the load, or stress at the middle of the bar, exceeds 

 a certain amount, the set resulting from one or more applica- 

 tions of the load on any one day is not only still discernible 

 on the following day, but the actual result may be that the 

 middle of the bar may be lower than at the close of the 

 observations on the previous day, Sach effects were observed, 

 in the experiments with white pine, when the load was suffi- 

 cient to produce a longitudinal strain on the upper or lower 

 fibers of 500 lbs. per square inch ; and in the experiments with 

 the steel bar, resting edgewise on its supports, when the strain 

 on the outer fibers amounted to 1500 lbs. per square inch. 



8. Eepeated applications of the same load, from day to day, 

 are attended with an indefinite augmentation of the residual 

 depression of the middle of the bar, if the load exceeds a 

 certain amount When a smaller load is similarly applied, 

 the set attains after a few days to a maximum, and subse- 

 quently subsides more or less. The load answering to the 

 critical point here referred to, is obviously the maximum 

 safe value for a variable load that can be applied, with an 

 indefinite number of repetitions, to the bar. In the case of a 

 white pine stick (3 in. by 3 in., and 4 ft. long) the experi- 

 ments show it to be less than I the theoretical breaking load. 

 Under repeated applications of 500 lbs. (or about J the theo- 

 retical breaking weight) the set steadily increased from day to 

 day— that is, the middle of the stick became more and more de- 

 pressed—during the entire period (seven days) that the pro- 

 longed effects were noted. Under daily repetitions of a load 

 equivalent to Vo the breaking weight, the depression increased 

 for three days, and after another interval of three days the 

 stick had recovered its original line. The depressions here re- 

 ferred to are those which obtained on the morning of each day 

 just before the first application of the stress on that day. 



9. In connection with the phenomena of set which have 

 been signalized, it is important to note that during any interval 

 in which a bar was kept under a transverse stress, the resulting 

 deflection commonly experienced a continual variation. In 

 general the deflection increased as the strain was prolonged, 

 ^ut the deflection of the steel bar in some instances diminished, 

 ^nder the prolonged strain. This unusual result was appar- 

 ently dependent on some molecular condition of the bar, in- 

 duced by previous strains. The comportment of the wrought 

 ^fon bar, as regards varying deflection under a continual strain, 

 was not particularlv examined. . 



is also noteworthy, in this connection, that the deflection 



bjected. The 



