COMPARISON OF EARTHQUAKE DIAGRAMS. 63 



the former Avas given by Horizontal Pendulum Seismograph of some- 

 what modified form, but similar in principle to the first. The hori- 

 zontal motion of the ground was magnified nearly six times in this 

 instrument, and a larger smoked glass plate of slower rate was em- 

 ployed. It is to be observed that the driving clock did not move at a 

 perfectly uniform rate, but was slighly retarded towards the end. The 

 circles are divided up to the eightieth second so as to f.icilitate the 

 comparison of the two diagrams. 



The vertical motion, which is given on the outermost of tlie three 

 circles, Avas registered by Ewing's Vertical-Motion Seismograph ; it has, 

 for its astatic mass, a heavy bob suspended from a horizontal axis with 

 an automatically changeable leverage. Ihe magnifying ratio of this 

 instrument is 1 to 8, or the motion of ground is magnified eight times 

 on the record ; the outside of the circle corresponds to up-motion and 

 inside to down-motion of the ground. 



Horizontal Motion. — In order to compare the two records of the 

 horizontal motion, Tlates VIII and IX. may be examined conjointly. 

 It will be observed that the plates revolved in opposite directions as 

 indicated by the directions of tlie arrows. 



The records begin at 0, but they are quite feeble till the end of 

 the fifch second ; then both the North-South and the East-West 

 components suddenly exhibited vigorous movements ; allowing for 

 multiplication introduced by the recording levers ( 1 to 4 in Plate VIII. 

 and 1 to 6 in Plate IX.) the displacement on the earth surface at the 

 sixth second was 1.4 mm. from South to Noi'th and 0.9 mm. from 

 East to AVest. When these two components are compounded together, 

 they give a resultant of 1.8 mm. in the direction of N. 33° W. and S. 

 33° E. The complete period at this point is approximately 0.8 

 seconds. 



Later on tliere are eonstant changes of phase relations in the 



