220 



REPORT — 1898. 



Figures like the above, representing the length of seismic waves, 

 although especially for the large waves we can rely upon the data for 

 velocity and period, must yet be accepted with great caution. For the 

 earthquake of October 28, 1891, as recorded in Tokio, it would appear 

 that seismographs were tilted through an angle of about one-third of a 

 degree, whilst the actual height of the waves was about 10 mm. If these 

 measurements, referred to symmetrically, formed wave-surfaces, the con- 

 clusion is that the lengths of the waves did not exceed 20 or 40 feet ; the 

 difference between which and, say, 1,600 feet is so great that all confidence 

 in the determination of wave-lengths is apparently destroyed within an 

 epifocal area, or, to be more precise, within five or six miles of an origin. 

 Where waves can be seen rolling down a sti'eet, we are here at least cer- 

 tain that the distance from crest to crest of an earth-wave is measured by 

 10 or 20 feet rather than by hundreds or thousands of feet. 



2. On the Velocity of Propagation of Large Waves. 



From the table on p. 221, where we find the length of arc along which 

 motion may have travelled, the velocity of the preliminary tremors along 

 such a path and the duration of their movements, which is the interval of 

 time by which they outraced the succeeding large waves, it is easy to 

 calculate the velocity with which these waves were propagated. The 

 results of such calculations, together with results obtained from somewhat 

 different data by von Rebeur-Paschwitz and Dr. A. Cancani, are given 

 in the following table : — 



Velocities of Large Waves in Km. per sec. 



3. On the Character of Earth-xcaves after having travelled Great Distances. 



The following remarks are based on records of earthquakes obtained 

 at distances from their origin so great that movement of the ground could not 

 be felt, whilst ordinary seismographs failed to indicate any movement of the 

 piers on which they rested. Many of them, for example, refer to seismo- 

 grams obtained in Europe or England of earthquakes which originated at 

 places so far distant as Japan. 



Fi-eliminanj Tremors. 



Velocity.— In the Report for 1897 (p. 173) a table is given of the 

 highest apparent velocities with which the preliminary tremors of about 

 seventy disturbances have been propagated over or across arcs of great 

 circles on the earth's surface. These arcs have varied in length from 

 about 2° to 156°. The observations on arcs of from 2° to 18° and from 

 70° to 85° have been fairly numerous. For arcs of intermediate length 

 the observations were only three or four, but inasmuch as these take up 



