I 



THE DEPTH OE AN EARTHQUAKE CENTKUM. 215 



A table similar to this has been compiled by Lasaulx.^ 



With the exception of the determination for the two 

 last disturbances these calculations have been made with 

 the assistance of the method of Seebach, which depends, 

 amongst other things, on the assumptions of exact time 

 determinations, direct transmission of waves from the 

 centrum, and a constant velocity of propagation. 



Admitting that our observations of time are practically 

 accurate, it appears that the other assumptions may often 

 lead to errors of such magnitude that our results may be 

 of but little value. 



From what has been said respecting the velocity with 

 which earth disturbances are propagated, it seems that 

 these velocities may vary between large limits, being 

 greatest nearest to the origin. 



If we refer to Seebach's method, we shall see that a 

 condition of this kind would tend to make the differences 

 in time between various places, as we recede from the 

 epicentrum, greater than that required for the construc- 

 tion of the hyperbola. The curve which is obtained 

 would, in consequence, have branches steeper than that of 

 the hyperbola, and the resultant depth, obtained by the 

 intersection of the asymptotes of this curve with the 

 seismic vertical, indicates an origin which may be much 

 too great. 



Another point worthy of attention, which is common 

 to the method of Mallet as well as to that of Seebach, is 

 the question whether the shock radiates directly from 

 the origin, or is propagated from the origin more or less 

 vertically to the surface, and then spreads horizontally. 

 We know that earthquakes, both natural and artificial, 

 may be propagated as undulations on the surface of the 

 ground, and that the vertical motion of the latter, as 



* Das M'dbeben i^on Herzogenrath, <^e., p. 134. 



