491 



shock at certain places, and points out the combinations, having 

 reference to surface configuration chiefly, producing such from a 

 single central impulse. 



Under the section I, the whole of the preceding information is 

 combined, to deduce the dimensions, form and subterraneous position 

 of the focal cavity, which the author shows to have been a curved 

 lamellar cavity or fissure of about 3 geographical miles in depth by 

 9 geographical miles in length, with an inclined vertical section, and 

 a mean focal depth (or depth of its central point of surface) of 5J 

 geographical miles below the sea. 



In section m are discussed, upon the data of hypogeal increment of 

 temperature (as supposed to be ascertained from deep mines and arte- 

 sian wells), the necessary temperature of the focal cavity, and the in- 

 tensity of the force that acted within it to produce impulse, assuming 

 that to have been due to steam at high tension, either suddenly 

 developed or suddenly admitted into a fissure rapidly enlarged by 

 rending. 



n. Deduces the amplitude of the wave and the work stored up in 

 it on reaching the surface, and compares the former with the observed 

 amplitudes. 



o. Deduces the velocity of transit of the wave of shock upon the 

 surface, from the most trustworthy of the observations of time at 

 various localities. These are found to correspond with considerable 

 exactness, and give a transit rate of between 700 and 800 feet per 

 second, as that at which the wave form was propagated from point 

 to point, differing with change of formation by amounts stated. 



p. Deduces the velocity of the wave itself, i. e. that of the wave 

 particle, which is shown to have been in round numbers between 13 

 and 14 feet per second (in the direction of the wave-path). A 

 remarkable relation is pointed out between this velocity and that 

 recorded for the earthquake of Eiobamba, the greatest whose effects 

 have been observed. The height due to the velocity of this wave is 

 to the altitude of Vesuvius as that due to the velocity recorded of 

 the Riobamba wave is to the mean height of the volcanic shafts of 

 the Andes, and more especially to the height of the volcanic vents 

 nearest to Riobamba. The author points out that the direct altitude 

 of a volcano is the true measure of the volcanic and seismic energy 

 beneath it, and not its volume, which is a measure both of energy 

 and time combined. 



