April 4, 1^595] 



NA TURE 



519 



tained the co-operation of observers in selected localities, each 

 of these should be furnished with a similar instrument, and if 

 possible receive personal instruction as to its installation and 

 working. If this is done, then an inexpensive type of apparatus 

 may be employed, which in an ordinary foundation will yield 

 results not much inferior to those obtained from more elaborate 

 arrangements, which subsequently it may be thought desirable 

 to establish. 



Althou};h it may only be possible to minimise the effects of 

 tremors, the records of these over extended areas may perhaps 

 present new features. Other movements which are likely to 

 be noted, but which will not influence the recording of move- 

 ments resulting from distant earthquake;, will be diurnal and 

 other periodic dis| lacements of the pendulums. The records 

 of these, together with those of local earthquakes, could hardly 

 fail in adding to the knowledge we possess about earth 

 movements. 



The principal object of the proposal made in the foregoing 

 remarks, is to determine the velocity with which earthquake 

 motion is propagated over the surface of the earth, and po.-sibly 

 through its interior. If it is established that vibratory motion 

 is transmitted with a measurable velocity through the earth, it 

 will be difficult tn overestimate the value of the knowledge 

 we shall have gained. The rigid scrutiny of the records 

 bearing on this latter point have to be left to European 

 observers. 



At present the « riier is engaged in drawing up a report upon 

 Ihe state of our knowledge respecting the velocity with which 

 earth vibrations and waves are transmitted through rock and 

 earth, and in making experiments to determine a form of simple 

 instrument which shall be not only sensible to slight changes 

 of level, but which is also capable of recording vibrations o( 

 small amplitude. 



Since writing the foregoing, which was printed for circulation 

 amongst a few of my friends interested in this branch of earth 

 physics, I have received NATt;RE o( Decetnber 27, 1894, in 

 which, on p. 208, Dr. E. von Rebeur-Paschwitz gives a description 

 of the remarkable disturbance of June 3, 1893, to which I have 

 already briefly referred. In )uly of that year I «ent photographs 

 of this record to various acquaintances in Europe. One of 

 these, together with a description of the same, because it was 

 illustrative of a great numt^er of unfelt earthquakes which I had 

 recorded, was sent wi'h the fourteenth report to the Briiish 

 Association on seismological work in Japan during 1893-1S94. 

 This report is, I believe, now in the press. The object in calling 

 attention to this matter is to show that thisdisturbanc, wherever 

 it originated, was also pronounced at places far distant from 

 Sirassburp, Nicolaiew and Birmingham. 



Mr. C. Davison, who writes the introduction to the description 

 of the European records of this earthquake, makes a brief 

 reference to the Hesirahility of having a lew »-ell-choser stations 

 in various paits of the world where earth ])uIsalions might be 

 recorded — a matter on which I have had considerable corre- 

 spondence with my friend Dr. E. von Rebeur-P.ischwitz. At 

 present Mr. Davison considers that Europe is fairly well 

 provided with instruments. Instruments are certainly fairly 

 numerous, but at the same time in many instances ihey vary in 

 their sensibilities and also in their objects. Judging from the 

 report of the Committee on Earth Tremors to the British 

 Association in 1S93. i>. 294, it would stem that the instruments 

 in Birmingham and Edinburgh are arranged to lie unaffected by 

 rapid tremors, and register "slow earth lilts only," while the 

 tromomet' rs of Italy are, I presume, constructed to record what 

 this name inplies. Whether they are able to record " elastic" 

 tremors is a dehatatile matter. A very gnod illustration of 

 what the heterogeneitv of the instruments at present empfiyed 

 in Europe leads to, is given by Mr. Davison in the records of 

 two earthqtiakec, contained in the report of the above-mentioned 

 Committee for 1894, which may have had velocities of propaga- 

 tion varying between I and 12 km. per second, according to the 

 type of instrument from which records were ob'aned. The 

 apparent explanation of these anomalies is that different 

 instruments have lecorded dilTerent phases of motion, and for 

 this reason I have been led to say that it is not likely ihit our 

 present knowle'lge will be increased until there is greater 

 uniformity in thr methods of observation. 



Mr. Davison's remark that "in Japan Prof. Milne's 

 tromometer (as described in British Association Report, 1892, 



pp. 207-209) leaves little to be desired," requires qualification. 

 As a " tremor " recorder it is excellent.but even as such I have im- 

 proved it by reducing its length to 30 mm. and its total weight to 

 0-39 grms. Unfortunately however, because it is a tremor re- 

 corder, its movements are such that even on a photographic film 

 only one metre distant, it may during a severe "tremor storm " 

 give a trace which appears as a band two inches in breadth, which 

 eclipses any effects due to dis'ant earthquakes, for the recording 

 of which it is therefore useless. At various times I have 

 experimented with at least a dozen of such contrivances, which 

 from the nature of their construction have necessarily short 

 periods of vibration, and are therefore not sensible to slight 

 lilting. Although I condemn these instruments for the record- 

 ing of distant earthcjuakes, I must admit that I am indebted to 

 a pair of them for having first made visible to me the diurnal 

 wave. 



In the Seismological Journal, vol. iii. p. 60, a sketch is given 

 of a horizontal pendulum, such as I have used to record the 

 daily wave and unfelt earthquakes since November 1893. One 

 of these, which has a boom 5 feet in length, has usually a 

 period of about 50 or 55 seconds. Altogether I have six sets 

 of such apparatus with photographic recording surfaces, 

 together with one or two others, which are read once per day. 



These have been installed at and given records from eighteen 

 localities — in caves, in the solid rock, in an underground 

 chamber, and on substantial columns rising from the natural soil. 

 Although these instruments are exceedingly bad forms for 

 tromometers, by these experiments — each of which continued 

 over several months, during which time continuous records were 

 obtained— much was learned about the localities where "tremor" 

 effects might be avoided. 



To solve the problem under consideration it does not seem 

 necessary to have an instrument sensible to less tilting than I ', 

 and it is certainly undesirable to have one sensible to the 

 phenomena called earth tremors or microseismic disturbances, 

 which appear to have the character of earth pulsations. What 

 is required is an instrument which is susceptible to the tilting 

 produced by the undulatory slow travelling quasi-elastic dis- 

 turbances of an earthquake, and at the same time is sensible to 

 Ihe minute, and possibly truly elastic vibrations which, both in 

 Japan and Europe, outrace the moie pronounced movements. 

 The first object, and to some extent the second, is certainly 

 attained by many instruments in Europe. From the records 

 given by the long pendulum of Dr. Agamennone, of which I 

 have not ha<l an opportunity to see a lull description or draw- 

 ings, I take it that for the preliminary vibrations, the pendulum 

 acts as a steady point, relatively to which the movements are 

 magnified by means of a pointer arranged like that of a seis- 

 mograph. The larger movements, which have periods of about 

 16 seconds, may he due to the slow heeling over of the pen- 

 dulum following the motion of the supporting to«er. About 

 these latter movements we already know a great deal, and their 

 velocities of propagation, as determined by the most accurate 

 methods in Japan, do not materially differ from the rate at 

 which the same disturbances continue on their journey to 

 Europe. 



What we most require is an investigation of the velocities of 

 propagation of the elastic movements which apparently go from 

 Japan 10 Europe in 15 or 20 minutes. 



If such phenomena exist, and if the European records are 

 correct, their existence is a reality, the instruments to record 

 their repetition must be sensible to small but rapid vibrations ; 

 and for the results to be comparable, these instruments must not 

 only be similar in construction, but they must be similarly 

 adjusted and similarly installed. 



Within a few days the writer will have completed two instru- 

 ments differing only in their size, which may be described as 

 conical pendulum sei-mographs in which the multiplication 

 relatively to their centres of oscillation will be adjustable from 

 about 20 to 40. The registration will as usual be photographic. 

 It is hoped ihat because the multiplication is large, and because 

 everything is as light as possible, the preliminary tremors 

 (elastic vibrations) may be recorded; while because the booms 

 are long, it is not unlikely that the sensitiveness to tilling will 

 be sufficient to record the slower waves. They will be tested 

 upon a foundation the diurnal tilling of which is known, and 

 where it is also known that tremors (earth pulsations) are 

 seldom met with. 



John Milne. 



NO. 1327, VOL. 5 1] 



