64 EARTHQUAKES. 



US that a still higher power above 12 might have detected 

 still earlier tremors. 



When gunpowder was the explosive, the observers 

 noted that the disturbance observed in the mercury took 

 a much longer time to reach a maximum than it did when 

 dynamite was employed. 



It was also observed that explosions fired beneath deep 

 water gave a higher velocity than similar explosions which 

 took place beneath shallow water. In the latter case 

 much of the energy was probably expended in throwing a 

 jet of water into the air. 



Another point which was observed appears to have 

 been that the rate varied with the initial shock. Thus : — 



Peet per second 



400 lbs. of dynamite gave 8,814 



200 „ „ 8,730 



70 ,» powder (deep) gave .... 8,415 



Also it is probable that the rate of a wave diminished 

 with its advance. For, 



Feet per second 



200 lbs. of dynamite gave for 1 mile . . 8,730 



„ „ „ „ 6 miles . • 5,250 



50,000 „ „ „ 8 „ , . 8,300 



„ „ „ „ 13| „ . , 5,300 



Greneral Abbot's general conclusions are : — 



1. A high magnifying power of telescope is essential 

 in seismometric observations. 



2. The more violent the initial shock the higher is the 

 velocity of transmission. 



3. This velocity diminishes as the general wave ad- 

 vances. 



4. The movements of the earth's crust are complex, 

 consisting of many short waves first, increasing and then 

 decreasing in amplitude ; and with a detonating explosive 

 the interval between the first wave and the maximum 



