L'L'L' I'i'ii'. K. Pearson. OH ik* Kinetic Accumulation qf 8tr 



of years, and partly to irregularities, physical and formal, i?i the 

 surface of the ground. 



Another source of irregularity affecting previous observations of 

 this sort, namely, thermometer errors arising from the uncertainty as 

 to the temperature of the liquid in the long stems of the mercury 

 or alcohol thermometers, does not in this case apply ; and if other 

 errors peculiar to the platinum thermometers exist, they seem to be 

 confined within much smaller limits. 



" On the Kinetic Accumulation of Stress, illustrated by the 



of Impulsive Torsion." By KARL PEARSON, F.R.S., Profe- 

 of Applied Mechanics, University College, London. Received 

 May 29, Read June 21, 1900. 



(Abstract.) 



1. It is usual in engineering practice to double the value of the 

 stresses, calculated statically, when a live load comes onto a girder ; and 

 further various empirical laws, such as those due to AVohler, are adopted 

 in the case of repeated loading to measure the effective resistance of a 

 structure. While these methods, practically adopted, show very clearly 

 that there is a just appreciation that loading varying with the time 

 differs in its nature very considerably from purely permanent loading, 

 they yet fall considerably short of the defmiteness required from the 

 theoretical standpoint. Occasionally it must be confessed that they 

 would fail even from the practical standpoint were it not for the large 

 factor of safety usually adopted. 



So soon as a live load comes onto a girder, even without impulse, 

 vibrational terms arise in the strains, and the same thing occurs 

 also in the parts of machinery subjected to external forces changing 

 with the time. The discussion of the strains in a girder due to 

 a rolling load was first undertaken by Sir George Stokes in 1849,* 

 and his results have been considerably extended in later papers 

 by Phillips, Renaudot, Bresse, and de Saint- Venant.t The latter has 

 further dealt with a considerable number of problems of what I 

 have elsewhere termed wm-impul*ir? /'>//////" ,+ as well as a variety 

 of cases of impulsive resilience in the case of bars receiving longi- 

 tudinal or transverse impacts. The numerical results of Saint- 

 Venant's papers, as well as his graphical representations, hardly seem 



See ' History of ElaMicity,' vol. 1, arts. 1276 and 1417. 



t IMC. cil., vol. 2, art?. 372 382. 



J Loc. rif., vol. 2, arts. 355357. 



Loc. cit., vol. 2, arts 401 414. (For the history of the subject see art. 341.) 



