368 



NATURE 



[Feuruarv 1 6, 1899 



The Density of the Matter Composing the Kathode 

 Rays. 



In a note which appeared in Nai i"RE of January 19, I in- 

 dicated a method by which an approximate limiting value could 

 be deduced for the density of the matter composing the 

 kathode rays. The result arrived at was that this must be small 

 compared with 10 '^ grams per cubic centimetre. This esti- 

 mate seems to be confirmed by results recently published by 

 E. Riecke (IVied. Ann., 66, p. 954) on the reaction-pressure 

 exerted on the movable kathode-vanes of an electric radio- 

 meter. A theoretical investigation leads the author to the re- 

 sult, that this pressure is equal to ;««-, in the notation of my 

 former note ('«, mass per unit volume ; h, velocity of the par- 

 ticles). His numerical results give an average of about '04 dynes 

 per square centimetre for the pressure, and he deduces for a, 

 theoretically, the value 9'6 x 10', or, say, 10' roughly. Using 

 these numbers, vi comes out about 4 x lo^-". 



(Queen's College, Belfast, February 11. \V. B. MORrON. 



Earthquake Echoes. 



An earthquake disturbance, as recorded at a station far re- 

 moved from its origin, shows that the main movement has two 

 attendants — one which precedes, and the other which follows. 

 The first of the.se by its characteristics indicates what is to follow, 

 whilst the latter in a very much more pronounced manner re- 

 peats at definite intervals, but with decreasing intensity, the 

 prominent features of what has passed. Inasmuch as these 

 latter rhythmical but decreasing impulses of the dying earth- 

 quake are more likely to result from reflection than from inter- 

 ference, I have provisionally called them Echoes, .\lthough I 

 see an explanation for the orderly arrangement and features of 

 the precursory vibrations, it is sufficient if I confine my remarks 

 in this note to the reverberations which apparently succeed an 

 earthquake. 



If it can be shown that our world resounds with earthquake 

 echoes, hypotheses at once suggest themselves as starting-points 

 for new investigation. 



It is, therefore, of importance that before the idea of earth- 

 quake reverberations should be crystallised, the evidence we 

 have of the supposed vibrational repetition should be carefully 

 scrutinised, and that opinions should be expressed respecting 

 the interpretations of observations like the following. When an 

 earthquake is comparatively small, and has originated as a .single 

 effort at no great distance (one or two thousand miles) from the 

 observing station, the seismogram shows a single set of pre- 

 liminary tremors, of short duration, a single set of pronounced 

 vibrations corresponding to an irregularly delivered originating 

 impulse, and finally a series of concluding vibrations which rise 

 and fall in value every three or four minutes. That which ap- 

 pears on a seismogram as a two-blow earthquake terminates with 

 dual reinforcements. .\s illustrative of this, I may refer to the 

 Isle of Wight seismogram of the .South Indian Ocean earthquake 

 of August 31, 1S98, copies of which have been sent to the mem- 

 bers of the Seismological Investigation Committee of the British 

 Association, and to their various co-operating stations We have 

 apparently here two large disturbances followed by pairs of 

 echoes. If we closely examine the first pair of these responses 

 (the second pair being t03 small to exhibit details), we find 

 that its subdivisions roughly agree in character with those ob- 

 servable in the collection of movcnunts which make up the 

 primary impulses. Calling the first maximum I, and its follow- 

 ing echoes r, i", and i'"; and the second maximum 2, and 

 its following echoes 2', 2", and 2'", the time intervals expressed 

 in minutes between these various phases are 



1 to 1' = 12. r to i" = 8. i" to r" = 4. 



2 to 2' = 10. 2' to 2" = 7-5. 2" to 2"' = 3-5. 



In considering these intervals it must be remembered that i 

 is greater in amplitude and period than 2, whilst I' is similarly 

 greater than l", &c. Now, seismological observation appears 

 to point to a result that is difficult to accept, viz. that the 

 smaller the amplitude of a wave, and the shorter is its period, 

 the higher, apparently, is the velocity of its prop.agation. 

 Should this be so, then it follows, as is indicated by the above 

 series of intervals, that the smaller echoes should return more 

 quickly than those which precede them. A much more certain 

 observation is made when this earthquake is regarded as result- 

 ing from a single impulse, and what has been treated as the 

 second maximum is examined as its echo. We then see that 



NO. 1529, VOL. 59] 



the five crests, constituting what has been called the first blow 

 or impulse, are repeated in what has been called the second blow 

 by a five-crest echo, the intensity of each component cor- 

 responding with that of its primary, .\fter this we get another 

 five-crest group, followed by two groups each of four crests, 

 beyond which point rhythmical recurrence is lost. A very good 

 illustration of what may be multiple echoes is found in the 

 Isle of Wight seismogram for June 29, 1898. This is a very 

 { large earthquake, which probably caused the whole of the 

 ! earth to pulsate, and its preliminary tremors indicate that it 

 • originated at a very great distance. It had a duration exceed- 

 [ ing three hours. The main disturbance shows more than 

 I fourteen maxima of motion which have a fairly symmetrical 



arrangement to the right and left of a central dividing lint-. 

 I Between these first movements and the first set of responses, 

 which commence suddenly, a faint but very uncertain likeness 

 j may be seen. When, however, we compare the responses, 

 i amongst themselves they apparently show a repetition in form 

 j and a uniformity in their time of recurrence that can hardly be 

 j the result of accident. To facilitate such a comparison two 

 I series of these concluding vibrations are here photographically 

 I reproduced, the first series being placed above the second. 



It will be noted that the triangularly-headed echo i' is not 

 unlike l " ; its spherically-formed successor 2' is repeated in 2" ; 

 and so we may continue through the series until we reach the gourd- 

 formed 6' and 7', reflected in corresponding shape by 6" and 7'. 

 Other points of likeness may be seen between 4 and 5 and be- 

 tween 5 and 6. I may add that if the photogram had been 

 made longer, then three groups of waves would have preceded 

 l', which would correspond in form and time with three groups 

 preceding i". 



The time intervals between these corresponding groups are 

 respectively as follows: from l' to I ', 2' to 2', ^;c. : 30, 31, 

 3°'5> 3'> 3'> 29 and 28 minutes. We here appear to be deal- 

 ing with a series of vibrational groups, each of which look 

 almost exactly half an hour to travel to and fro between two 

 reflecting surfaces or districts. If the waves were compressional 

 in character, the distance between these surfaces would be 

 about Sooo kms. ; but if they travelled with the velocity of the 

 v/aves of shock, this distance would be reduced to something 

 under 3000 kms. From their period and amplitude it is 

 probable that the distance lies between these values. 



The main point at issue, and the one tn be answered before 

 we enter into further speculations, is whether seismograms show- 

 ing this musical repetition can be interpreted in the manner 

 here suggested. The concluding vibrations of an earthquake 

 have usually been regarded as a disorderly mob of pulsatory 

 movements resulting from spasmodic impulses, which gradually 

 grow feebler as the activity at a seismic centre becomes ex- 

 hausted. The (|uestion before us is whether an earthquake dies 

 by a process analogous to repeated and irregular settlements of 

 disjointed materials, or whether it is simply a blow or blows 

 which come to an end with musical reverberations inside the 

 world. For the present my opinion inclines to the latter, 

 and I see in the earthquake followers the likeness of their 

 parents. 



The observational confirmation of the existence or non- 

 existence of these echoes requires a special arrangement of 

 apparatus, installed in a dry, well-ventilated room, having a 

 proper site, and free from tremors. In the dark, damp stable 

 where I work, I regret to s.ay that the frosty nights have 

 brought with them vigorous and persistent tremors, and as a 

 good observing .season has now commenced, beautiful seismo- 

 grams are being spoiled. The last to suffer was that of a 

 magnificent set of waves which arrived from Mexico on the 

 night of January 24. J. ins MlLNE. 



February 1. 



