68 s. SEKIYA. COMPAEISON OF EARTHQUAKE DIAGEAMS. 



is sensibly stable. If it had been at all affected by the continuous 

 shaking of the earth it must have ultimately swung with its own 

 period of oscillation; however it did not swing, as is clearly proved on 

 the diagram by the shortness of the periods of undulation. 



To find out experimentally the steadiness of horizontal pendu- 

 lums, Prof Ewing placed two of them on a shaky table, and by rigidly 

 fixing the bob of one instrument to a neighbouring wall by means of 

 a bracket, and letting the other bob free, he moved the table in such a 

 manner as would resemble the earthquake motion. The free and fixed 

 instruments produced waves which were almost alike to each other. 

 This shows that the steady mass of the free pendulum remained 

 stationary during the whole of the varied motion. 



Allowing then for certain instrumental errors which cannot in 

 all cases be eliminated, we have, in the agreement of records produced 

 by similar instruments during the same earthquake, or during artificial 

 shaking, evidence which conclusively preuves the accuracy' and trust- 

 worthiness of the Horizontal Pendulum Seismograph. 



