THE POSITION AND EXTENT OF THE SEISMIC VERTICAL 65' 



From Calcutta we have one observation made on the direction of 

 oscillation of the water in a tank ; in an observation of this kind accu- 

 racy is not so attainable as in the more simple case of a pendulum, such 

 as the chandelier above mentioned, and consequently it is not surprising- 

 that the direction given falls some way from the seismic vertical as 

 already defined. 



At Sylhet the direction was noted in which the trees were set swing- 

 ing, and this, when plotted, also falls among the intersections of the first 

 class. 



At Supakatti the observation of S. 5° W. evidently does not point 

 towards the seismic vertical ; but we must not hastily conclude that this 

 is altogether due to error of judgment, it being possible that the wave 

 was reflected from the eastern portion of the Naga hills, which would 

 give it a nearly north and south direction. 



Thus we see that of the observations of the second class two agree 

 well with those of the first class, one fairly well, and one is altogether 

 at variance with them. 



To sum up, then, there are 30 intersections of the first class and 6 of 

 the second, in all 36, falling on an area 40 miles long by 4 or 5 broad; 

 but of these it will be advisable to omit those obtained from the hospital 

 at Yeddo and those from Cachar, where the direction seems to have been 

 more or less modified by the proximity of the river banks. The intersec- 

 tions' are comprised in an area about 3 or 4 miles broad and from 

 20 to 30 miles long, running approximately E. 15° N. and W. 15° S. y 

 situated in north latitude 26° and east longitude 92° 40", lying on the 

 northern borders of the Jaintia Hills. 



The shock, we may conclude from this, probably originated in a 

 fissure about 20 miles long running underneath the tract mentioned in the 

 last paragraph and at a considerable depth below the surface. 



Such is the direct evidence on this point. As regards the position 

 of the seismic vertical, it is indeed satisfactory; but as to the shape of the 

 geismic focus, it is incompatible with that obtained from other sources. 

 As the discussion of this point will necessitate reference to matters as 

 yet unmentioned, I shall defer it to another page. 



o ( 65 ) 



