NATURE 



[December i, 1898 



periods of 2 to 2 i sec; movements of i to rj mm. range, 

 which constitute the greater part of many records, have 

 periods of about i sec. The record ends in simple 

 vibrations of periods rj to 4 seconds, like waves of the 

 ocean after a storm. 



It is to be presumed that magnetic and other messages 

 are constantly passing to us through the earth, which, if 

 we could only read them — if we knew anything about the 

 cause of terrestrial magnetism, for example — would tell us 

 something about the inside of our habitation. Earth 

 shivers which have travelled from Japan to England seem 

 to be the only messages coming to us through the earth's 

 body which we are able to understand, and which can 

 give us at present any knowledge of what that body is like. 



X'ibrations from gunpowder explosions in the earth are 

 found to travel faster if the explosions are more violent. 

 Mallet's experiments gave velocities of from 250 to 500 

 metres per sec. The velocities deduced from the Hell- 

 Gate explosion vary from 1300 metres per second to 6200 

 More sensitive apparatus records higher velocities, be- 

 cause it has been acted upon by the smaller tremors, and 

 these travel faster than others. Prof. Milne gives a list 

 of earthquake speeds actually observed. 



Counting time from the first record anywhere and 

 distance from the place of this first record, measuring 

 either by an arc on the earth's surface, or by a chord, we 

 may look upon the following speeds as not unusual. 



The more decided state of motion which arrives with 

 more or less of definiteness of transition after the tremors, 

 travels at a much slower rate ( I to 3! kiloms. per sec), and 

 hence the time of duration of the preliminary tremors 

 tells us the distance to the origin. Hence when a seis- 

 mograph record is made in the Isle of Wight, the 

 distance through which the motion has travelled may 

 generally be estimated with some accuracy, and hence 

 also the violence of the initial disturbance may be 

 guessed at. 



It seems to be out of the question that the preliminary 

 tremors have travelled on the earth's surface either as 

 Lord Rayleigh's surface waves or as distortional waves 

 or condensational waves. We can only understand their 

 propagation if we imagine the stuff in the earth at great 

 depths as having cubic elasticity very much greater than 

 that of the best steel, but it is not so easy to understand 

 how such enormous elasticity can be possessed by it. 

 Those students of nature who assume that the behaviour 

 of rock on the earth's surface enables them to speak with 

 easy certainty as to the beha\ iour of rock at enormous 

 pressure and temperature, must surely find some diffi- 

 culty in understanding the above observations, just as 

 NO. 1518, VOL. 59] 



there are celestial phenomena which must surely distur1> 

 the equanimity of the unimaginative persons who apply 

 their farthing rush-light laboratory experience without 

 modification or reserve to all the phenomena of the- 

 universe. 



The author describes the instruments used now to 

 note or accurately record or measure the motion of the 

 ground or of any point in a structure when an earth- 

 quake or shiver passes. The invention of a good 

 instrument for each kind of motion has exhausted much 

 mathematical and inventive genius. It will be found 

 that Prof. Milne is wonderfully fair to other inventors. 

 There seems still to be some doubt as to how much of 

 the motion recorded as horizontal motion of the ground 

 is perhaps something different, an eft'ect of tilting of the 

 instrument. The study of the whole subject has un- 

 doubtedly added to our knowledge of vibration phen- 

 omena, and I should think that this book will be found 

 of great value by young engineers in charge of electric 

 light stations, where the most important problem of the 

 present time is how to protect citizens from the annoying 

 vibration of reciprocating steam engines. At some 

 future period it may enter the minds of manufacturers of 

 engines that the lineal descendant of Newcomen's slou 

 reciprocating pumping engine is not perhaps the best 

 kind of quick speed motor to use in a thickly inhabited 

 district, and that although passengers by steamer must 

 put up with such a nuisance because there is no for- 

 bidding law, the manufacturers of electric energy may 

 easily be compelled to reform their methods of working. 

 The study of all such vibration can be carried on only 

 by the use of instruments such as Prof. Milne describes 

 They have already given useful information in showing 

 how wasteful of coal a badly balanced locomotive may 

 be ; in picking out badly constructed parts of the per- 

 manent way in railways, in detecting loose parts Id 

 bridge construction, and, more important than any of 

 these, they are now giving us information as to the 

 dangerous vibrations of some bridges when trains pass 

 over. 



One important result that seems to be gradually 

 establishing itself in the minds of observers is that th^ 

 number of shakes s per day (vigorous enough for record 

 on a particular kind of instrument) at any place at the 

 time / days after a large earthquake, and at the distance 

 /; miles froin the epicentre, is a not very complex sort of 

 function of n and /. From the numbers given by Prof. 

 Milne, 1 venture to say that the law for the after-shocks 

 of the 1891 earthquake is something like 



an expression that is worth some study as representing 

 fairly well the result of observation. An enormous 

 amount of labour has been given to the search for 

 periodicity in earthquakes. It does seem that seismic 

 and barometric maxima coincide, and possibly because 

 of this, or because of snow accumulation, there is greater 

 seismic activity in winter than in summer, although more 

 destructive earthquakes seem to occur in summer. The 

 much looked for dependence of seismic activity on the 

 moon's position is not yet established so well as the 



