174 



NA TURE 



\_Junc 19, 1884 



'MEASURING EARTHQUAKES* 

 II.— Results. 



IN this paper a short account will be given of the chief 

 results of two and a half years' observations in the 

 Seismological Observatory of the University of Tokio. 

 The first instruments to be successfully used were the 

 horizontal pendulum, or rather a pair of horizontal pendu- 

 lums writing a multiplied record of two rectangular hori- 

 zontal components of the earth's motion on a revolving 

 plate of smoked glass, and also a very long common 



pendulum. The duplex pendulum, an astatic vertical- 

 motion seismograph, and other instruments which have 

 been mentioned in the former article, were added later. 1 



The earliest records were those of five small earth- 

 quakes in November 1SS0.' 2 In the first of these the vibra- 

 tion of the ground lasted continuously for \\ minutes, 

 and no fewer than 150 complete oscillations could be 

 I counted in the record. The shaking began feebly, 

 ' speedily rose to a maximum, fluctuated irregularly, and 

 ; died out very gradually. The greatest movement from 

 1 side to side was less than one-third of a millimetre. Both 



in amplitude and in period the successive waves were far 

 from equal. A rough idea of the greatest velocity and 

 greatest acceleration was, however, obtained by treating 

 the greatest movement as a simple harmonic vibration, 

 with a period of three-fifths of a second. This gave 

 l'6 mm. per second for the greatest velocity, and 164 mm. 

 per second per second for the greatest acceleration, showing 

 that bodies attached rigidly to the earth's surface must have 

 experienced a horizontal force equal to about one-six- 

 hundredth of their own weight. In three of the five earth- 



quakes recorded in the same month the greatest range of 

 motion was less than one-fifth of a millimetre. In all of 

 them there were many and unequal vibrations, but in 

 none was there any single impulse prominently greater 

 than the other movements. 



Later observations showed that these were fairly repre- 



1 For a fuller account of the methods and results of these observations 

 the writer may be permitted to refer again to his memoir on Earthquake 

 Measurement, published as No. 9 of the Memoirs 0/ the Science Depart- 

 ment of the University 0/ Tokio. 



* Inscribed in the Transactions of the Asiatic Society 0/ Japan, 

 1 I. i.\. p. 40. 



