TIME OF EARTHQUAKE : RATE OF PROPAGATION. 299 



from the most central part of the epifocal zone being 1°45 / , or 195 kilometres. 

 Within such an epicentral distance the velocity of propagation of the first prelimi- 

 nary tremor is probably 5 or 6 kilometres per seconds. If we assume the velocity 

 to be 6 kilometres per second the time taken by the seismic waves in travelling 

 the distance of 195 kilometres would be about 32 seconds. Now, as the magneto- 

 graph is not so sensitive as a seismograph, the time moment recorded at Dehra 

 Dun might correspond to the commencement of the principal portion, the dura- 

 tion of the total preliminary tremor being probably some 20 seconds. Thus the 

 time of earthquake occurrence at the epicentre may approximately be taken as 

 Oh. 50m. 38s minus about 50 seconds, or Oh. 49m. 48s. G. M. T. [which is 

 6h. 10m. 48s. Madras time]." 



With reference to Captain Thomas' note, however, I pointed out. 

 that with regard to his deduced times if we neglect 



Criticism oi Cap- . . 



tain Thomas' note! tne -Dehra Dun time as being disputed we have by 

 difference of time and space (using his own figures): — 



Difference of time, Kodai and Barrackpore . 319 sees. 



Do Toungoo and Barrackpore . 262 „ 



Difference of radial distance from centre : — 



Kodai and Barrackpore . . . 554 miles. 



Toungoo and Barrackpore . . 568 „ 



Rate in first case 554 -f 319 which is . . 1*74 miles per sec. 



2nd case 554-262 . . . 2*17 ,, 



Now these radial distances and times should at that distance from 

 the epicentre give a uniform rate (approximately) which they do not, 

 the difference amounting to nearly J mile per second. Also buth are 

 too low according to his own estimate. So that if from them we cal- 

 culate backwards to the point of origin, namely, 1,498 and 1,512 miles 

 respectively, we get 6h. 7m. 39s. and 6h. 9m. 26s. respectively for the 

 time at the epicentre. 



The above reasons make me hesitate to accept the results of the 

 magnetogram readings as in any way contributing to our knowledge of 

 the true time of the sensible shock. 



In reply to this criticism Captain Thomas quite agreed to the prin- 

 ciple of deducing rates by the difference method leaving Dehra Dun 



