ON SEISMOLOGICAL INVESTIGATIOX. 



60 



When examining the above table it must be remembered that pendu- 

 lum C is oriented at right angles to A and B, that the stiffness of the 

 column carrying A is slightly greater than that carrying B, and also that 

 these two pendulums have not always been so adjusted as to have the 

 same sensibility to the effects produced by tilting. It must also be borne 

 in mind that preliminary tremors are but small movements, and their 

 visibility will vary with the width of the photographic trace, whilst many 

 of the entries referring to amplitudes of 25 millimetre are markings so 

 slight that they might easily escape detection. 



When the periods of A and B were 17 seconds (572 to 583 and 607 to 

 609) out of thirteen records, the differences in time at which these instru- 

 ments commenced to record did not exceed 1 minute in seven cases, 



