210 



REPORT — 1S98. 



describes in some detail the disturbances he noted in connection with the 

 Assam earthquake o£ June 12, 1897. 



Dines' Pressure Tube Anemometer Chart did not show any trace of 

 atmospheric disturbance. The barograph trace was, liowever, disturbed, 

 the maximum effect being about 1 min. later than the maximum disturb- 

 ances in the declination and vertical force magnetograms. These instru- 

 ments are of the Kew pattern. The needle of the declinometer has a 

 period of 5-33 seconds, and the disturbance it suffered is here shown 

 enlarged about 2^ times. The time of vibration of the needle of the 

 horizontal force magnetograph, which shows an equally large disturbance, 

 is 8 seconds. The disturbance in the vertical force magnetograph, which 

 is also pronounced, continued over three minutes. There are fourteen 

 fairly regular waves in 29 mins.; that is, the magnet which, if mechani- 

 cally disturbed and allowed to return to rest, would do so in double 

 swings of 5'33 sees, came to rest with periodic movements, each of which 

 had a duration of two minutes. In Europe the earth-waves from the 

 earthquake had periods of from 10 to 15 sees., and it is likely that when 

 they passed Bombay their periods would be about five seconds. It is 

 difficult to understand how a movement of this description would result 

 in the displacements recorded; and, as Mr. Moos points out, it is equally 

 difficult to see why earth- waves could mechanically cause a change in the 

 scale reading of this type of instrument. His conclusion is that the 

 seismic convulsion was in some way the cause of a magnetic action, every 

 seismic wave having its companion effect in a magnetic wave. • 



The following is a summary of disturbances of magnetic needles at 

 various observatories by the shock of June 12, 1897 (see Earthquake No. 

 105, p. 204) : — -Time at origin, about 11 hrs. 4 mins. a.m. ; arrival of pre- 

 liminary tremors in Europe, about 11 hrs. 17 mins.; arrival of large 

 waves in Europe, about 11 hrs. 47 mins. 



Magnetographs at Lyons, Perpignan, Pola, Vienna, Uccle, and Lisbon 

 were not disturbed. 



The barograph disturbance at Bombay was at 11 hrs. 14 mins. to 

 11 hrs. 21 mins., with max. at 11 hrs. 13 mins. ; the electrometer dis- 

 turbance at Batavia was at 11 hrs. 16 mins. (exact). 



