76 



REPORTS ON THE STATE OF SCIENCE.— 1915. 



hand, it was found tliat the freedom of the dumb-bell set up undamped 

 oscillations of the light spot ; so Mr. Shaw clipped the mass rigidly to the 

 boom. The moment of inertia of the dumb-bell about its centre of mass 

 was measured by hanging up the dumb-bell by a bifilar suspension and 

 observing its time of oscillation. In this way the radius of gyration I of 

 the whole moving system about the point R was found to be 11 -66 cms. 



The magnification for impulses of lateral displacement is j = jy-^= 257. 



There is an unexplained discrepancy of 10 per cent, between this figure 

 and 282, which might be sought for in the neglected action of the multi- 

 plying lever. This was not quite perfectly balanced. In the tilting 

 experiment it remains untilted. 



In drawing fig. 2 the mean of 257 and 282 has been used. 



Magnification for Long-continued Sinusoidal Waves of Lateral 

 Displacement. — When the period is infinitely short, this is the same as 

 the magnification for impulses. The diminution of the magnification 

 with increasing period of the earth-wave is determined from the period 

 of the pendulum and the degree of its damping, according to the well- 

 known formula (Walker's ' Modern Seismology,' page 5). To obtain 

 the damping, a piece of a soft iron nail was attached to the boom. A 

 small solenoidal coil of wire was fixed on the pillar so that the iron was 

 half inside the coil. By passing a momentary electric current through 

 the coil the boom was set in motion. Care was taken that the applied 

 force did not continue for more than a small fraction of the quarter-period 

 of vibration. As the solenoid had no fixed iron core, there were no after- 

 effects. As one has to measure the ratio of successive swings, this ratio 



fi^riod or the Earth Wal^e in seconds 



Fig. 2. 



Milne-Shaw Seismograph, No. 3. July 26, 1915. 



The full-line curve gives the magnification for long-continued sinusoidal 



waves of lateral displacement. 



Undamped free period of pendulum 9'9 sees. 



Damping ratio of successive swings on opposite sides of zero 45 : 1. 



The crosses are from the Galitzin instrument. 



