ox THK KARTHQUAKE AND VOLCANIC PHEXOMEXA OF JAPAN. 159 



impulse, travel at a liisjlier rate than those resulting from an effort of 

 lower intensity. 2. Near to an origin, the velocity of propagation is greater 

 tlian it is between points at a distance. '■'>. After a disturbance has 

 decreased in its speed of transmission it may be accelerated, and this 

 acceleration cannot be with certainty attributed to its having entered a 

 more elastic medium. 4. As an earthquake radiates, it is preceded by a 

 series of minute tremors, the velocity of propagation of which is certainly 

 A^ery much higher than that of the main disturbance. 



(a) Artificialli/ produceadisturhances. 1. Experiments o/'Mr. R. Mallet, 



In the experiments of the late Robert Mallet conducted at Killiney 

 Bay, Dalkey, and Holyhead (' British Association Report,' 1861) the initial 

 impulse was caused by the explosion of cliarges of gunpowder. The 

 electrical contact which caused the explosion released a chronograph 

 which was stopped by a^i observer directly he saw, by means of a 

 microscope magnifying 11-39 times, an agitation caused by the resulting 

 waves in a dish of mercury. After corrections for the intervals of time 

 thus noted, in round numbers the results obtained were as follows : — 



In wet sand . . . . 

 In discontimious pranitc . 

 In more solid granite 

 In granite at Holyhead (mean) . 



0'25I km. per. sec. 

 0-:-598 „ „ „ 

 0-507 „ „ „ 

 0-371 ,. „ „ 



The charges of powder employed varied between 25 lb. and 12,000 lb., 

 and with but one exception it was clearly shown that the velocity of wave 

 propagation increased with the force of the initial impulse. For example, 

 at Holyhead the relationship between the quantity of explosive and the 

 resulting velocities was as follows : — 



2. E.iperimcnts by General II. L. Abbot. 



In 1885 when Flood Rock was destroyed by the explosion of 240,397 lb. 

 of rack-a-rock and 48,537 lb. of dynamite, the most distant observing 

 station was 182-68 miles off". The instant of the explosion was noted at 

 all the points of observation by means of electrical connections and 

 chronographs, while the arrival of the fir.st tremors and their duration 

 was recorded by observers who watched the disturbance of an image 

 reflected from the surface of mercury. 



The Hallet's Point observations, where the initial impulse was due to 

 the explosion of 50,000 lb. of dynamite, and others made in connection 

 ■with .subaqueous explosions at the school of submarine mining at Willet's 

 Point, were conducted in a .somewhat similar manner. In the following 

 table, which has been drawn up from the scattered writings of General 

 Abbot, the A'elocities have been reduced to uniform units : — 



