EARTHQUAKE MEASUREMENT AT MTYAKO. 193 



period seems to remain approximately constant in tlie principal 

 and the end portions. These undulations existed more or less 

 also in the preliminary tremor, the mean of the average period 

 being 1.1 seconds in the (EW) and 1.0 second in the (NS). 



For the V.M., the mean of the average periods of slow 

 undulations was about 0.8 second. 



It is remarkable that the average period of ripples remained 

 roughly constant in the different earthquakes here considered, 

 never varying much from 0.1 second. It is possible that these 

 ripples were preceded by, or superposed upon, very minute 

 vibrations of still shorter periods. 



43. The range of motion. The maximum ranges of motion 

 in different earthquakes are given in the 9th column of the 

 Table. The greatest ripple of 7.4 mm. occurred in Eqke. No. 11, 

 the next greatest of about 5 and 6.4 mm. occurring respectively 

 in Eqke. No. 4 and Eqke. No. 23. The greatest slow undula- 

 tions, which probably occurred in these two earthquakes, were 

 unfortunately confused by the proper oscillations of the pendulums 

 serving as the " steady masses " of the seismograph. As an 

 example, in which this element of motion was satisfactorily 

 measured, Eqke. No. 3 may be mentioned, the maximum range 

 of motion in question being 9.1 mm. 



The range of V.M. was invariably smaller than that of the 

 corresponding H.M., the maximum motions of the two components 

 being, for ripples, in the average ratio of 0.20 : 1 ; and for 

 slow undulations, in the average ratio of 0.21 : 1. 



44. The direction of motion. The motion in the principal 

 portion was, with the exception of one case or two, always greater 

 in the (EW) than in the (NS). For the preliminary tremor, 

 however, no such peculiarity was found, the ratio of the ranges 



