SEISMOMETKY. 



23 



wooden arm w carrying two sliding pointers H n, resting 

 on a glass plate placed on the top of the pendulum. 

 These pointers are for the 

 purpose of giving the fric- 

 tional resistance before re- 

 ferred to. If this friction 

 plate is smoked, the fric- 

 tion pointers will write 

 upon it records of large 

 earthquakes independently 

 of the records given by the 

 proper index, which only 

 gives satisfactory records 

 in the case of shocks of 

 ordinary intensity. Cross- 

 ing the inside of the pen- 

 dulum R there is a brass 

 bar perforated with a small 

 conical hole at M. A stiff 

 wire passes through M and 

 forms the upper portion of 

 the index i, the lower p3r- 

 tion of which is a thin 

 piece of bamboo. Fixed 



upon the wire there is a small brass ball which rests on the 

 upper side of a second brass plate also perforated with a 

 conical hole, which plate is fixed on the bar o crossing 

 the box. 



If at the time of an earthquake the upper part of the 

 index I remains steady at M, then by the motion at o, the 

 lower end of the index which carries a sliding needle at 

 G, will magnify the motion of the earth in the ratios 

 M o : o G. In this instrument o G is about 17 cm. 



The needle G works upon a piece of smoked glass. In 



■iimi 



c m o , 



iiiiiiiiiimiiiiiiiiiimmiim^^ 

 Fig. 2. 



