SEISMOMETEY. 27 



mating with the period of the earthquake. The result 

 of these experiments showed that it was usually those 

 with a slow period of vibration which were the most 

 disturbed. 



Bracket Seismographs, — A group of instruments of 

 recent origin which have done good work, are the bracket 

 seismographs. These instruments appear to have been 

 independently invented by several investigators: the 

 germ from which they originated probably being the well- 

 known horizontal pendulum of Professor Zollner. In 

 Japan they were first employed by Professor W. S. Chaplin. 

 Subsequently they were used by Professor Ewing and Mr. 

 Gray. They consist essentially of a heavy weight sup- 

 ported at the extremity of a horizontal bracket which is 

 free to turn on a vertical axis at its other end. When the 

 frame carrying this axis is moved in any direction except- 

 ing parallel to the length of the gate-like bracket, the 

 weight causes the bracket to turn round a line known as 

 the instantaneous axis of the bracket corresponding to 

 this motion of the fixed axis. Any point in this line may 

 therefore be taken as a steady point for motions at right 

 angles to the length of the supporting bracket. Two of 

 these instruments placed at right angles to each other 

 have to be employed in conjunction, and the motion of 

 the ground is written down as two rectangular components. 

 In Professor Ewing's form of the instrument, light pro- 

 longations of the brackets form indices which give 

 magnified representations of the motion, and the weights 

 are pivoted round a vertical axis through their centre. 



In the accompanying sketch B is a heavy weight 

 pivoted at the end of a small bracket C A K, which bracket 

 is free to turn on a knife-edge, K, above, and a pivot A, 

 below, in the stand s. At the time of an earthquake b 

 remains steady, and the index P, forming a continuation 



