RIGIDITY OF ROCKS AND HYSTERESIS FUNCTION. 31 



The figure in PI. XVI. gives further illustrations of the effects of 



torsional hysteresis in causing- a loop to he formed on the curves of 

 twist when the couple experiences several cyclic changes with differ- 

 ent amplitudes and centres. Here it is to he seen that the greater 

 the amplitude of the cycle, the more the loop tends to become 

 vertical. 



Starting from a strained state, a specimen was twisted cyclically, 

 the centre of each cycle being different from that of the others. The 

 result is shown in PI. XIX., all of the loops produced by the several 

 • •yrles being nearly similar to one another, though they are widely 

 different in their dimensions. We see, however, that when the 

 amplitude of the cycle is given with regard to the amount of the 

 applied couple, the loop whose centre is further from zero-stress is 

 more horizontal than the other. 



Other particulars with respect to the natures of the torsional 

 hysteresis will be learned from the several figures in the annexed 

 plates which have been carefully drawn from experimental results. 

 Comparing the figures in Pis. XVII. and XVIII. with those in PI. 

 XV., we can not but be struck with the very close coincidence 

 between theory and experiment. Here it is necessary to remark that, 

 though a piece which has been twisted at least once can not be really 

 neutral, yet the process of demagnetization in the case of magnetic 

 hysteresis suggests a means by which any twisted piece can again be 

 brought to a practically neutral state. One example of this process is 

 shown in Fig. 48, PI. XX. To begin with, in any of the above 

 experiments, this process was applied to get rid of the latent traces 

 of previous history which the specimen may have acquired. 



To give a general idea of the comparative magnitudes of the 

 rigidity of several rocks, some of the stress-strain curves for different 

 rocks are drawn, Fig. 49, PI. XXL, on one and the same scale as 



