FORESHOCKS AND AFTERSHOCKS. 361 



1 every half-hour, and it is clear that records for one every 2 or 3 minutes 

 would certainly appear in the earthquake forms if we allow that the 

 average clock was not accurate to within half-an-hour (a perfectly 

 reasonable supposition in outlying stations in India). 



In support of the above assumption may be mentioned here the 



Frequency of records of the H. F. magnetograph at Dehra Dun. 



aftershocks as shown That instniment reco rded 15 aftershocks between 



by the magneto - 



graphs at Dehra 6-21 a.m. and midnight, the several times being : — 

 ^ un - 6-21, 6-29, 6-31, 6-46, 7-39, 7-51, 8-56, 9-4, 13-41, 



13-59, 15-50, 17-55, 18-30, 21-59, 23-59. The average here exhibited 

 is about 1 every hour, although 4 are recorded in the first hour, and 

 there are some notable gaps, as between 9-4 and 13-41. Dehra Dun is, 

 however, rather far away from the larger centre, although on the other 

 hand the magnetograph probably recorded shocks that were not 

 sensible to men. 



Having then just cause to believe that the severally recorded shocks 

 in that crowded early part of the list are by reason of imperfect timing 

 inextricably mixed up with one another, it becomes manifestly impos- 

 s ble to do anything more with the data than to conclude that over 

 such and such a general area including such and such big towns, after- 

 shocks occurred about 1 every 30 minutes during the day of the big 

 event. 



With the date April 11th the list begins to thin so considerably, as 

 regards frequency, that aftershocks in widely separated localities can 

 now be reasonably correlated. For instance the shocks reported at 

 Gurdaspur, Simla, Mussoorie and Landour, Dehra Dan, Manglaur and 

 Mandi between the reported times 10-30 a.m. and 12. midday are 

 probably one and the same shock. We may therefore conclude that it 

 was felt over the area including those places, but negative evidence 

 that it was not felt elsewhere has not much value. 



On April 15th again, there is the same sort of evidence for a shock 

 about 6 a.m. felt at Mussoorie and Landour, Rurki, Kulu and Muzaf- 

 farnagar. At Rurki it was distinctly felt and doors rattled, whilst in 

 Kulu it is reported as the worst since the shock of the 4th. 



