216 EAKTHQUAKES. 



testified by the records of well-constructed instruments, 

 has no practical connection with the depth from which 

 the disturbance originated. 



In cases like these, the direction of cracks in buildings, 

 and other phenomena usually accredited to a normal 

 radiation, may in reality be due to changes in inclination 

 of the surface on which the disturbed objects rested. 

 When our points of observation are at a distance from the 

 epicentrwm of the disturbance which, as compared with 

 the depth of the same, is not great, calculations or ob- 

 servations based on the assumption of a direct radiation 

 of the disturbance may possibly lead to results which 

 are tolerably correct. The calculations of Mallet for the 

 Neapolitan earthquake appear to have been made under 

 such conditions. 



For smaller earthquakes, and for places at a distance 

 from the seismic vertical of a destructive earthquake, the re- 

 sults which are deduced from the observations on shattered 

 buildings, and all observations based upon the assumption 

 of direct radiation, we must accept with caution. 



Another error which may enter into calculations of 

 this description is one which has been discussed by Mallet 

 at some length. This is the effect which the form and 

 the position of the focal cavity may have upon the trans- 

 mission of waves. 



Should the impulse originate from a point or spherical 

 cavity, then we might, in a homogeneous medium perhaps, 

 regard the isoseismals as concentric circles, and expect to 

 find that equal effects had been produced at equal distances 

 from the epicentrum. Should, however, this cavity be a 

 fissure, it is evident that even in a homogeneous medium 

 the inclination of the plane of such a cavity will have 

 considerable effect upon the form of the waves which 

 would radiate from its two walls. 



