90 



EARTHQUAKES. 



whicli Mallet had at his disposal, are but rude approxi- 

 mations to the truth. Two interesting facts are, how- 

 ever, observable: the first being that the velocities for 

 the earthquake of 1761 are much higher than those 

 obtained for the earthquake of 1755 ; and, secondly, that in 

 both cases the velocities as determined from the observa- 

 tions of ships at sea closely approximate to each other, 

 in all cases being nearly the same as that with which a 

 sound wave would travel through water. 



The great differences in transit velocity obtained for 

 different earthquakes is a point worthy of attention. 



Seebach's velocity is a true transit velocity, and its 

 determination is dependent on the assumption that the 

 shock radiated from the centrum and not from the e'pi- 

 centrum, Seebach's method is explained when speaking 

 about the determination of origins. 



Some interesting observations on the velocity with 

 which the earthquake of October 7, 1874, was proj)agated, 

 are given by M. S. di Eossi.^ 



One assumption is that the disturbance radiated from 

 an origin to surrounding points of observation, whilst 

 another is that the disturbance followed natural fractures, 

 the direction of which is derived from the crest of certain 

 mountain ranges. These velocities are as follows, Maradi 

 being at or near the origin of the disturbance : — 



Velocity in feet per second with, 

 direct radiation 



Velocity in feet per second by propagation 

 along mountain chains 



Modigliana . . 820 

 Bologna . . .656 

 Forli. . . . 874 



By the Yalley of Marenzo 

 „ „ Saveno 

 „ „ ]\Iontone 



1,080 

 1,080 

 1,080 



Modena . . .618 



„ „ Panaro 



/ 1,080 



\ 984 



540 



540 



Firenze . . ., 273 

 Compiobbi . . 328 



„ „ Sieve . 



>> »> J) • • 



* Meteorologia Endogena, 1. p. 306. 



