EARTHQUAKE MOTION DEDUCED FROM EXPERIMENT. 59 



direction of motion was at right angles to that line. The 

 nature of the resultant motion will be gathered from fig. 

 10, which is taken from the records drawn by the rolling 

 sphere seismograph at a distance of 50 feet, 100 feet, and 

 200 feet respectively from the point where the ball struck 

 the ground. The direct or normal vibrations reached the 

 instrument first, and were followed at an interval depend- 

 ing on the distance of the instrument from the origin by 

 the transverse vibrations. From the records of these two 

 sets of vibrations as separated by the bracket seismographs, 

 combined with the known rate of motion of the glass 

 plate, the velocity of transmission was found to be, for 

 normal vibrations 446-438 feet per second, and for trans- 

 verse vibrations 357-353 feet per second. 



The effect of the hill in cutting off the disturbance 

 seemed to be slight, but the direction of the vibrations 

 which ascended the side was mostly transverse. The 

 pond, on the other hand, seemed completely to cut off the 

 disturbance, which, however, gradually crept round the 

 side, so that only a comparatively small triangular area 

 was in shadow. 



The amplitude of the vibrations diminished directly as 

 the distance increased for some distance from the origin, 

 but at greater distance the rate of diminution seemed to 

 be slower. The transverse vibration seemed to die out 

 less quickly than the normal vibrations.^ 



These experiments were afterwards very considerably 

 extended by the author. In these later experiments 

 charges of from one to two pounds of dynamite were 

 placed in bore-holes of various depths and exploded by 

 means of electricity. The results obtained confirmed the 

 conclusions already arrived at from the former experiments. 

 The experiments on velocity, however, seemed to indicate 

 > See Phil. Trans. B. i% Part III. 1882. 



