68 



REPORT — 1902. 



VI. On the Comj^arison of Records obtained from three Horizontal Pendu- 

 lums at Shide, the Natural Periods of winch have from time to time 

 been altered. 



Tlie instruments referred to are the photographic recording pendulums 

 described on p. GO. Their records are compared in the following table. 

 The type of pendulum which records east-west motion is referred to by 

 the letter A. The pendulum with the short boom, also recording east- 

 west motion, and forming part of the Yarrow instrument, is referred to as 

 B, whilst the north-south boom of the same is called C. 



Pendulum A has been kept with a period of 17 seconds, whilst the 

 periods of B and C have from time to time been changed. 



The numbers in the first column refer to different earthquakes re- 

 corded between January 1 and June 30, 1902, as entered in the Shide 

 Register (see Circular No. 6). In the next three columns are the differ- 

 ences in minutes at which A, B, and C respectively commenced to move. 



The first entry for Earthquake No. 572 means that A and B show 

 commencements of movement 4 minutes later than C, beneath which 

 there is a zero. 



Amplitudes are also referred to under three columns marked A, B, 

 and C. They indicate half the complete range of the maximum motion. 

 Values less than one millimetre refer to the thickening of the line, and 

 indicate half its width. 



Durations of the different earthquakes are given in the last three 

 columns. They are expressed in minutes : — 



