THE DEPTH OF AN EARTHQUAKE CENTRUM. 219 



Mallet showed that there was probably considerable 

 truth in such a supposition by appealing to the results of 

 actual observation. The pressure gauge of the Neapolitan 

 district would be Vesuvius, the height of which has in 

 round numbers varied between 3,500 to 4,000 feet. One 

 of the most destructive earthquakes in this district — 

 namely, the one of 1857 — projected bodies with an initial 

 velocity of about fifteen feet per second. The Eiobamba 

 earthquake, which projected bodies with an initial velocity 

 of eighty feet per second, appears to have been the most 

 violent earthquake, so far as its impulsive effort is con- 

 cerned, of which we have any record. It occurred 

 amongst the Andes, where there are volcanoes from 16,000 

 to 21,000 feet in height. 



Comparing these two earthquakes together, we see 

 that the Eiobamba shock had a destructive power 5*33 

 times that of the Neapolitan shock, and we also see that 

 the Eiobamba volcanoes were about 5*33 times higher 

 than Vesuvius. The accordance in these quantities is 

 certainly interesting, and tends to substantiate the idea 

 that volcanoes are barometrical-like pressure gauges of a 

 district. 



Carrying the argument still further, Mallet says that 

 if the depth of origin of earthquakes were the same, then 

 the area of disturbance would, for like formations and 

 configuration of surface, be a measure of the earthquake 

 effort, and also some function of the velocity of the wave. 

 From this we may generally infer ' that earthquakes, like 

 that of Lisbon, which have a very great area of sensible 

 disturbance, have also a very deep seismal focus, and also 

 the greatest depth of seismal focus within our planet 

 is probably not greater than that ascertained for this 

 Neapolitan earthquake, multiplied by the ratio that the 

 velocity of the Eiobamba wave bears to that of its wave, 



