On the Transit-velocity of Earthquake-waves. 



231 



One very remarkable result is at once apparent on inspecting this 

 table — viz. that the transit rate tends to increase in velocity with the 

 increased quantity of powder fired ; in other words, that the loss of 

 velocity in the same rock is less in some proportion as the force of 

 the originating impulse is greater, and so its amplitude greater at 

 starting. This is seen if the experiments be arranged in the order 

 of increased weight of powder. 



No. of experiment 



2. 



3. 



1. 



6. 



4. 



5. 



Weight of powder 



2100 lbs. 



2600 lbs. 



3200 lbs. 



4400 lbs. 



6200 lbs. 



12,0001b. 



Uncorrected trans- 

 it velocities . . 



967-93 



977-26 



896-12 



996-11 



1173-87 



1210-79 



Experiment No. 1 forms the only apparent exception, and even 

 there the departure is not large. 



This fact, now for the first time (so far as the author knows) ex- 

 perimentally proved, appears remarkably in coincidence with the 

 theoretical researches of Mr. Earnshaw. 



The general mean transit velocity derivable from all the experi- 

 ments taken together gives 1176*407 feet per second for the rate. 

 The results, however, obviously form two groups — viz. Nos. 1, 2, 3 

 and 6 from the smaller, and Nos. 4 and 5 from the greater charges 

 of powder. The mean from the first four is 1088*5597 feet per 

 second; that from the two last 1352*1015 feet per second; and 

 taking a mean of means from both, we obtain 1220*3306 feet per 

 second as the mean transit velocity of propagation, in the rocks expe- 

 rimented on, of wave pulses due to the impulse of explosions of 

 not exceeding 12.000 lbs. of powder. 



The first mean from the smallest charges is that which must be 

 compared with the Killiney Bay experiments. It thus appears that 

 the wave velocity in highly contorted and foliated rock is very low, 

 and is intermediate between the transit rate in wet sand and in dis- 

 continuous granite, or 



In wet sand = 824*915 feet per second. 



In contorted and stratified rock, quartz, and slate = 1088*559 

 feet per second. 



In discontinuous granite = 1306*425 feet per second. 

 In more solid granite = 1664*574 feet per second. 

 The general mean obtained, 1220*33 feet per second, or 13*877 

 statute miles English per minute, co-ordinates, as might be expected, 

 with the carefully made deductions of Noggerath and of Schmidt 

 from the actual earthquakes of the Rhine and of Hungary, as well 

 as with those of the author from the great Naples earthquake of 

 1857. 



In experimenting with these great explosions the author was en- 

 abled to observe, by means of the seismoscope, that the advent of 

 the great wave of impulse (which was sometimes sufficient to make 

 the mercury sway visibly in the trough of the instrument) was pre- 

 ceded by rapidly augmenting tremors, quite like those which very 

 generally precede the great shock in natural earthquakes. 



