Reviews — Prof. John Milne — Earthquakes. 373 



and students desirous of learning more may consult a paper in 

 ■vol. xxxix. of the Quarterly Journal of the Geological Society ; and 

 the Transactions of the Seismological Society of Japan. Not the 

 least important application of those beautiful instruments has been 

 the observation of artificial earthquakes. Much curious and useful 

 information has thus been obtained. The effects of canals and 

 even ditches in arresting surface vibrations, though to some extent 

 previously known, are here more completely established. A result 

 of the highest scientific importance that seems looming in the possible 

 future is a true measure of the intensity at origin of a disturbance, 

 as is hinted at on page 62. One drawback to the value of these 

 experiments is that they are made solely with disturbances produced 

 by percussion or explosion, while we can hardly doubt that some 

 earthquakes at least, and probably very many, are due to causes 

 which have no resemblance to a blow. 



Interesting purely theoretical discussions will be found in various 

 parts of the book. The diagram in Chap. VII. illustrating reflection 

 and interference is clear and instructive, but readers should remember 

 that the energy of a reflected shock is probably greatly diminished 

 from the energy of incidence. The problem of deducing the origin of 

 an earthquake from observations of its time of occurrence at several 

 distinct points receives elegant treatment in Chapter X. All methods 

 described are vitiated theoretically by changes in velocity of pro- 

 pagation (which Prof. Milne himself has shown to exist), and practi- 

 cally, in such cases as our own Colchester Earthquake, by the 

 inexactness of most records of time. When these last can be trusted, 

 it is probable that the theoretical methods will approximate to the 

 true origin, if there be one, with sufficient accuracy for any practical 

 purpose. 



A well-known effect, often observed and often commented on, the 

 rotation produced in many objects by the earthquake, is discussed in 

 the chapter on ' Determination of Origins.' Preference is given to 

 an ingenious explanation credited to Mr. Gray. We cannot, however, 

 ourselves see that this is a very essential advance on the explanation 

 given by Mallet. Their common principle is that a resultant impulse 

 which does not pass through the centre of gravity will create in the 

 body rotation, and the point of the new suggestion appears to be that 

 in bodies on rectangular bases, this impulse must in general come 

 from one angle. The explanation is not applicable for circular bases. 

 Both seem insufficient in view of Meldola and White's observation 

 ("East Anglian Earthquake of 1884," p. 206), that chimneys were 

 in general rotated all in the same direction ; with which agrees Prof. 

 Milne's own remark on the stones of the Yokohama cemetery. 

 After all, what object is there in trying to dispense with ' vorticose 

 motion ' ? As we are dealing not with water, but with earth, the 

 only meaning of the phrase is that a vibratory motion is propagated 

 in a direction transverse to the direction of vibration ; the trans- 

 mission, in brief, of a wave of shearing strain. All instruments at 

 present in use only record the motions of a single point, and none of 

 them are able to distinguish between normal vibrations and trans- 

 versal such as these. 



