ox THE EARTHQUAKE AxVD VOLCANIC THEXOMENA OF JAPAN. 169 



mencement of movement by the more sensitive classes of instrument, the 

 records from which also give the lower values, if the arrival of the dis- 

 turbances is taken as being the time when they recorded jnaximum phases 

 of motion. 



As a last example of the different results which may be obtained from 

 the same record, I take that observed at Rocca di Papa on March 23, 

 1894. The shock originated beneath the ocean about 70 miles S.E. of 

 Nemuro, on the north-east coast of Yezo (Lat. 42° N., Long. 146° E.). 

 It was observed at Nemuro in Greenwich mean time at 10.20.45 a.m. 



The times at Rocca di Papa and the resulting velocities were as 

 follows : — 



First tremors, JO.36.0 G.M.T. 11-35 km. per .sec. 



Decided motion, 11.12.0 ,, .S06 

 Ma.ximum motion, 11.19.0 ,, 2-fi9 „ 



In Tokio it was observed at 10.27.40 G.M.T., after which four after- 

 shocks were noted, The average time difference between the oUservations 

 at Tokio and Nemuro was fi min. 43 sec, and the difference in their 

 distances from tlie origin is about GOO miles, from which an average 

 velocity of about 2-3 km. per second is calculated. If it is assumed that 

 the first tremors reached Rocca di Papa by direct radiation along a chord 

 or through the earth, then their velocity may be reduced to 8 or 10 km. 

 per second (see example on p. 149). 



3. Conclusions. 



Very many records might be added to those which have been given, 

 but it does not seem likely that, until we are in possession of a series of 

 records taken at long distances apart on the surface of our globe by means 

 of instruments ivMch are similar, which have sufficient sensibility to record 

 preliminary tremors, and which record upon surfaces moving sifficienthj 

 quickly to alloxv of accurate time determinations, that our present know- 

 ledge \yill be greatly increased. Because the waves of a disturbance change 

 in period as they travel, while one wave breaks up to form two or more 

 waves, and this even in ground which is apparently homogeneous, a given 

 earthquake may show as many velocities as there are waves between which 

 we choose to make measurements. What we know from experiments, and 

 what we should expect from a priori reasoning, is that the rate at which a 

 disturbance is propagated varies with the nature of the medium through 

 which it is ti-ansmitted. Experiments have shown that the vibrations fol- 

 lowing an artificial disturbance, where the initial impulse has been strong, 

 travel more quickly than those where the originating cause has been feeble; 

 also that there is apparently a higher velocity very near to an origin than 

 at a distance. The latter phenomenon seems to find confirmation in the 

 records of certain earthquakes. Although it may be difficult to interpret 

 the meaning of these latter observations, when we endeavour to find an 

 explanation for the existence of the long series of preliminaiy tremors 

 which are recorded at places nearly a quarter of the earth's circumference 

 from an origin, and which have apparently reached these places by travel- 

 ling at rates of 9 to 12 km. per second, the difficulties which confront 

 us are still greater. The next section is an attempt to explain these 

 phenomena. 



