NATURE 



[January 2, 1902 



at an ordinary temperature, or six hours at loo'', it has 

 returned to its transparent state. With Ford blotting- 

 paper the time of recovery is longer. On the other hand, 

 if a paper be made only slightly moist, this facilitates 

 the passage of the peroxide through it. If different 

 substances be dissolved in the water used for wetting 

 the paper it modifies the result obtained. With some 

 substances the paper is not permanently affected, but with 

 others— such, for instance, as alum— the paper remains 

 opaque. 



If paper be either written or printed on, the different 

 effects which are produced have been already described. 



•.,1 F 



-_-_ . ^-.i^ 



The ordinary writing inl<, allowed to dry on paper, 

 renders it perfectly opaque to the action of the peroxide 

 and retains this power for a very long time. The 

 ■direction of a letter written in 1801 shows the writing 

 with remarkable sharpness. The picture was produced 

 by placing a zinc plate behind the letter. Then with 

 regard to printing ink, it is a body which in itself is 

 active, so that it has only to be brought in contact or 

 in proximity to a photographic plate to give a picture. 

 Naturally the activity of the ink varies much in different 

 cases, and is in most cases capable of giving, not only a 

 picture where the ink is facing the plate, but the printing 





iA. >(.-/ V,.*- yj 



on the other side of the paper will also be depicted on 

 the photographic plate. Fig. 6 shows this very well, the 

 printing on both sides of the paper being very evident. 



This difference of the action of writing and printing 

 ink is well shown in the two pictures of an old cheque, 

 Figs. 7 and 8. No. 7 is simply an ordinary photo- 

 graph of the cheque, but No. 8 is a picture of the 

 same cheque produced by placing it on a photographic 

 , plate with a zinc plate behuid it ; the printing-ink has 

 ■become inactive, Init the writing ink is still able to 

 , prevent the hydrogen peroxide from passing through. 



W. J. Ru-SSELI.. 



NO. 1679, VOL. 65] 



IVHAT ARE SEISMOMETERS INDICATING? 



ONE thing which a modern seismometer does, and 

 does in a satisfactory manner, is to indicate the time 

 of arrival of the various phases of motion which con- 

 stitute an earthquake. With two similar instruments at 

 the same station, the time of commencement of a given 

 earthquake is practically identical ; but if the installa- 

 tions of the instruments are different — for example, if the 

 instruments rest upon piers of different heights and con- 

 struction — it will only be the pronounced phases of the 

 subsequent movements that can be identified and which, 

 therefore, can be compared. If our instruments, instead 

 of being in the same room, are 100 to 800 feet apart, the 

 only points in the two seismograms which can be identi- 

 fied will be the commencements and the pronounced 

 shocks, which latter are of rare occurrence (sec British 

 Association Reports, 1SS5). As a rule not only will the 

 general appearance of the seismograms be different, but 

 measurements will show the existence of differences in 

 period and amplitude. This being the case for seismo- 

 grams obtained at stations near to each other, what 

 coincidences can we possibly hope for in seismograms 

 obtained at stations located at a few, several hundreds or 

 several thousands of miles from each other ? 



Next, if we turn to a consideration of the character of 

 the earth movements which produce seismograms re- 

 lating to earthquakes the origins of which are at a great 

 distance, we meet with observations the explanation of 

 which is not simple. The first rapidly recurring tremors 

 that a seismograph records are regarded as elastic waves 

 of compression andjrarefaction. One reason for this belief 

 is that the observed velocities with which these pre- 

 cursors traverse either the surface materials or the 

 body of the earth are such as would be expected for this 

 particular form of wave in the media considered. 



Following these iforerunners by an interval of time 

 which increases with the distance of the observing 

 station from the earthquake origin are a series of more 

 pronounced movements, usually referred to as shocks or 

 large waves, about the character of which there have been 

 differences of opinion. For earthquakes originating 

 within a few hundred miles of an observing station we 

 find that the records of such waves obtained from bracket 

 seismographs are described as horizontal movements, 

 whilst those from spring-lever seismographs are referred 

 to as vertical components of motion. So long as the 

 latter records are not shown upon a seismogram the 

 former have always been regarded as described, their 

 apparent magnitude being dependent on the multiplica- 

 tion of the writing indices. When, however, in a register 

 we see entries for "vertical motion," neither the measure- 

 ments for this nor for the corresponding entry for horizontal 

 displacement can be relied upon. The reason for this 

 statement, first made more than ten years ago, is that 

 with severe earthquakes for 100 or more miles round the 

 epicentre we have a vast amount of evidence showing 

 that the ground is thrown mto a series of surface-waves. 

 These angular displacements cause horizontal pendulums 

 to swing from side to side, whilst the levers of lever 

 seismographs move up and down, the result being that 

 both types of instruments, instead of measuring com- 

 ponents of motion relatively to steady points, act as 

 indifferent clinograpbs. In consequence of these con- 

 siderations, at the end of iSgi I designed a clinometer 

 for earthquakes. Briefly, this consisted of a balance- 

 beam loaded at its two extremities, which when its frame 

 was tilted in a direction at right angles to its length was 

 assumed to retain its horizontality. .\ pointer like that 

 of an ordinary balance attached to this beam acted as a 

 steady fulcrum for the short arm of a light lever, the 

 outer end of which rested on a smoked glass surface. An 

 example of the seismograms giving the period and slope 

 of earthquake-waves obtained by this apparatus will be 

 found in the British .\ssociation Reports, 1893. 



