A MODEL SHOWING THE MOTION OF AN EAETH-PAETICLE. 



361 



Before going farther it ought to be stated that the origin of this 

 earthquake was in the S.W. in a narrow band of country, seven- 

 teen to forty miles from the spot where the recording instrument 

 was placed. 



The earthquake begins, as usual, Avith short-period tremors. 

 During the third second there appears for the first time a vigorous 

 horizontal motion, IST.W. and S.E., (that is, at right angles to the line 

 joining the origin of the disturbance and the instrument), accompanied 

 by a considerable vertical displacement. Both horizontal and vertical 

 motions then continue with great activity ; at the ninth second (Fig.. 

 I), the upward displacement of 1.3 mm. or y-/' is recorded, which 

 is the largest vertical motion during this shock. Synchronously 

 with it, there occurs the horizontal motion of 5 mm. or |" N.W. and 

 S.E., the complete period of oscillation in both being 1.5 seconds.. 

 Equally large vertical motion also appears at the tenth second. 

 Vertical motions are most marked during the first part of the disturb- 

 ance and give to Fig. 1 more striking features than to the other two. 



The maximum horizontal displacement of 7.3 mm., or nearly 

 ^", occurs later on from the thirty-third to thirty-fourth second with 

 the complete period of two seconds. Its direction is then nearly AY. 

 S.W. and E.K.E. or nearly in a line with the origin of the shock. 

 There is, however, no prominent vertical motion simultaneous with it. 



The directions of the principal horizontal motions in Fig. 3 are 

 S.E. and X.W., or transverse to the direction of the origin of the dis- 

 turbance, but it would be premature to draw thence any conclusion 

 as to the relations between the directions of the local movements of 

 the ground and the position of the origin, as the seismic waves are 

 influenced in their passage by a great many circumstances. 



In the portions of the disturbance exhibited in Fig. 2 only 

 few important up-and-down oscillations occur, as is shown by its^ 



