588 Transactions. — Chemistry and Physics. 



given as 2-42 barely, and that the previous rumbling lasted 

 for 12 seconds ; so that it was possibly at least 6 seconds 

 earlier. This seems to prevent one being influenced by the 

 times at Nelson and Bull's, which would be satisfied more 

 nearly by an epicentrum nearer E. S is sixty miles from 

 Wanganui and sixty-six miles from Wellington. 



The question remaining to be settled is the relation be- 

 tween the results obtained from the sets of observations A 

 and B. They are certainly the observations of two different 

 phases. Are they simply the observations of two different 

 maxima of disturbance, or do those of one set belong to the 

 normal vibrations and those of the other to the transverse 

 vibrations ? If the B times give the slower (probably trans- 

 verse) vibrations, starting from the origin (E or S) at the same 

 time as the quicker (normal) vibrations of A, then their 

 velocity of propagation was thirty to thirty- three miles 

 per minute (2,640 ft.-2,904ft., or 88,500 cm.-88, 600 cm., per 

 second). The difference in the velocities of propagation is 

 very marked ; but it must be remembered that the theoretical 

 value of the transit velocity of transverse vibrations at the 

 origin is less than that of the normal vibrations, and that 

 the amount of loss of the former is greatly affected by the 

 question whether the vibrations are along or across the planes 

 of lamination, while the loss of velocity of the normal vibra- 

 tions is scarcely affected by this at all. 



The calculated velocity for normal waves in granite (the 

 elasticity being that determined by Gray and Milne from 

 their experiments on Japanese rocks) is, according to them, 

 395,000 cm. per second ; the velocity of the transverse waves 

 219,000 cm. per second, or eighty-two miles per minute 

 nearly.* The latter depends on the rigidity modulus and the 

 density only ; and Gray and Milne's figure for the former 

 is very low (128 x 10*^ grammes per square centimetre). 

 Lord Kelvin's figures (157 X 10'^) gives, I find, about 

 258,000 cm. per second, or ninety-six miles per minute nearly. 



The actual velocity of the waves (B) when observed is 

 about one-third of the theoretical value for transverse waves, 

 and this is not an improbable value ; often as much as eleven- 

 twelfths of the theoretical value is lost in the initial stages ; 

 after that the velocity remains . tolerably uniform. (See 

 Button, "Charleston Earthquake"; and Ibbetson, "Elastic 

 Solids"). 



The explanation, then, that the A observations are those 

 of normal vibrations and B those of transverse vibrations is 

 quite a plausible one, if supported by other considerations. 

 No single velocity that can be assumed will agree with both A 



" Quart. Journ. Geol. Soc, vol. 39, p. 139. 



