58 s. SEKIYA 



eight observations were made ; in four cases the vertical motion 

 reached 0.02 m.m. The laro'est value 0.56 m.m. was observed in 

 the severe earthquake of February 22nd, 1880. In 1883 Prof. J. 

 Milne, in conjunction with Prof. T. Gray, made experiments on 

 artificial earthquakes. ^ A^ibrations then were caused by letting 

 fall a heavy Aveight from various heights or exploding dynamite in 

 holes made in the ground. His results were principally as follows. 

 (1) In the soft ground vertical motion appears to be a free surface 

 wave which outraces the horizontal components of motion. (2) 

 Vertical motion commences Avith small rapid vibrations and ends 

 with vibrations wdiich are long and slow. (3) High velocities of 

 transit of seismic waves may be ol^tained by the observation of this 

 component of motion. It is possibly an explanation of the pre- 

 liminary tremors of an earthquake and the sound })henomena. 



In the table are given the following quantities. 



i. — Maximum Motion (2r) or the largest range of the displacement 

 of the ground in each shock. 



2. — Complete Period ( ^ ) of the maximum motion or the time taken 

 to make a complete for-and-back motion of the ground. 



3. — Maximum Velocity ( -y ) of the ground, or v = -^ — . 



i^. — Maximum Acceleration = — . 



r 



The last two quantities were calculated Ijy assuming for con- 

 venience sake the motion of the o-found to be harmonic thouo^h 

 it is not exactly so in actual cases. 



o. — Direction of the maximum horizontal motion of the ground. 



6. — Duration of the earthquake, i. c., the interval of time from the 

 commencement to the end of the disturbance. It is almost 



1 Transactions of the Seismological Society Vol. VIII. 



