HuTTOX. — The Earthiuakc in the Amuri. 283 



being produced by them. It was also felt strongly in the 

 mines at Reefton. In order to obtain some idea of its depth 

 I tried Professor Milne's method of co-ordinates, but without 

 success. As each of the simultaneous equations is more or 

 less inaccurate, different results are obtained by combining 

 them in different ways, and I found, after many attempts, that 

 these results for velocity of transit and depth of centrum were 

 so discordant that I lost all confidence in them. The method 

 is a very good one when the data are sufficiently accurate ; but 

 it is not adapted for obtaining the most probable result from a 

 mass of incorrect data. 



We have seen that the effects of the shock were most 

 severe in the neighbourhood of Glynn Wye, so that probably 

 the angle of emergence was here between 45° and 55°, and this, 

 with the epicentruni situated sixteen miles away, would give 

 the depth of the centrum between sixteen and twenty-three 

 miles. When we have discussed the velocity of propagation 

 we shall be able to make another approximation to the 

 position of the centre of the centrum. 



Velocity of Propagation. 



Westport, Boatman's, and the epicentrum are nearly in a 

 straight line ; and, if d is the distance of the centrum from 

 Westport, and r/i its distance from Boatman's, t and ^i, being 

 the time taken by the wave to pass to each place respectively, 

 then we have, on the assumption that the velocity is the 

 same in both cases — 



d- — v't' ; r/i" = v'^ti- ; and t — ti = 2 minutes. 



• ^' ^ ^li 



■ ■ 7^ " 7? • 

 (1.) dit-dti = 0.) 

 (2.) t- h =.2.i 



Eliminating i'l from these two equations, we have — 



t 2./ 



d-di 



Also v = - . 

 t 



The values of d and di depend upon the depth at which w^e 



place the centrum, and consequently t and v depend upon it 



also. The following results are obtained for dilierent values 



of z, which is the depth of the centrum below the surface ; 



the distance of Westport from the epicentrum being sixty-two 



miles, and Boatman's thirty-six miles : — 



If ^ = 15, then t -- 5-15 and v = 12-39, or 1,090ft. per second. 

 If z ^ 20, then t ^^ 5-46 and v = 11-92, or 1,049ft. per second. 

 li z ^^ 25, then t --^ 5-81 and v = 10-74, or 945ft. per second. 



