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APPENDIX. 



In figure 1, let h h represent the surface of the earth, and A the focus at which, from 



whatever cause, an impulse of an explosive nature is communicated to the surrounding 

 rocks ; the particles of rock immediately surrounding A are driven asunder and then 

 return to their normal positions, thus originating a wave of elastic compression, which 

 spreads outwards in every direction from the focus, as is represented hy the concentric 

 circles surrounding the point A. Now, as the motion of the particles of rock is, except 

 where interfered with by want of homogeneity in the rock, always directly outwards 

 from the focus, it will be seen that the direction of motion of the particles is the same 

 at no two points of the area over which the shock is felt, and that at each point the 

 direction of motion points to the focus or point of origin of the wave ; if then, at any 

 two or more points, we can get the true direction of movement of the wave particles, 

 we can deduce the true position of the focus. 



The object of all seismological observations should be, primarily, to determine 

 both the true direction and velocity of motion of the particles set in motion by the 

 earthquake wave. This motion, which is at an angle to the horizontal, may be con- 

 sidered as being composed of a vertical and a horizontal component. For the deter- 

 mination of these two components, we have to depend on observations which may be 

 divided into two great classes — subjective and instrumental ; the first consisting of 

 those impressions or sensations which may be personally noted during the passage of the 

 earthquake shock ; the second, of those observations made with properly designed in- 

 struments ; while a third sub-division consists of observations, made after the passage 

 of the wave, of damage done to permanent structures. 



Beginning, then, with the subjective observations, I would say at once that all 



impressions of the direction in which the shock travels are, from a scientific point of 



view, almost, if not utterly, useless, for it is but seldom, and then mainly by accident, 



that these impressions agree with the facts. But, though guesses at the direction in 



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