202 REPORT — 1881. 



II. Observations to determine the nature of Tjarthquahe-motion. 



With the help of specially contrived seismographs, so far as my ob- 

 servations have hitherto gone, it is shown — 1st. That the actual horizontal 

 motion of an eaTth-particle at the time of an earthquake is very much 

 smaller than we anticipated from our senses, being seldom over a few 

 millimetres and often under one millimetre. 



2ndly. The backward and forward motion of the ground is very ir- 

 regular, both in regard^to space and time. 



Srdly. That there are seldom more than two or three complete vi- 

 lirations per second. 



These observations, I am pleased to say, have been confirmed by a 

 more complete series of records than my own, obtained by Professor 

 Ewing. 



4thly. A motion often takes place in more than one direction. Both 

 at the Yokohama station and the Tokio station records have been 

 obtained of two sets of irregular ellipses crossing each other nearly at 

 right angles, these ellipses being drawn by the pointer of a seismogi-aph 

 moving over a smoked glass plate at rest beneath it. 



For the same earthquake this diiference in direction may be experienced 

 at one station, but not at another, 15 or 20 miles distant. 



From records obtained it would seem possible that at Yokohama both 

 transverse and normal vibrations may be recorded, whilst at Tokio, six- 

 teen miles distant, only the former are recorded. As these latter obser- 

 vations need confirmation, I hope during the coming year to be able, 

 with the help of a few similar instruments placed at different stations, to 

 obtain records of a series of shakings, to show the relation between the 

 dying out of one set of vibrations as compared with the dying out of 

 another. 



Ill, The recording of Eartli-tremors. 



With the help of a specially- contrived instrument, by which a motion 

 of the earth equal to the ttmjtt^^ o^ ^"^ inch can be definitely recorded, I 

 am able to say that in Tokio there are very many small disturbances in 

 the ground, which are not registered with any of our ordinary seismo- 

 graphs. 



My object in recording these small motions is to see, first, whether 

 they are in any way connected with the larger motions which we call 

 earthquakes, just as the crackling of a stick is connected with its break- 

 ing ; and, second, whether there is any periodicity connected with these 

 small movements, it being possible, for example, that influences which 

 produce the tides in the ocean may perhaps be sufiicient to produce earth 

 cracMes, whilst they may not (excepting where they are, so to speak, 

 like the last straw upon a camel's back) produce an earthquake. 



As Yokohama appears to be nearer to the origin of many of our 

 earthquakes than Tokio, and also because, as compared with Tokio, it 

 stands upon the rock, I have quite recently taken two of my tremor 

 instruments there. One of them has been placed in the hands of Mr. W. 

 H. Talbot, and the other with Mr. H. Pryer, these gentlemen having 

 very kindly undertaken to keep a record of their movements. 



