EARTHQUAKES. 



123 



explained. A series of waves, somewhat similar in form to water- waves 

 (though differing in nature), actually passes beneath the observer. Of 

 course, when an object is on the front-slope, it will lean in the direction 

 of transit ; and when on the hind-slope, in the contrary direction. 



3. Circle of Principal Destruction. — In some earthquakes a certain 

 zone at considerable distance from the point of first emergence (epicen- 

 trum) has been observed, within which the destruction by overthrow is 

 very great, and beyond which it speedily diminishes. This has been 

 called the circle of principal destruction or overthrow. It is thus ex- 

 plained : The overthrow of buildings depends not so much on the 

 amount of oscillation as upon the horizontal element of the oscillation. 

 Now, the whole amount 



of oscillation is greatest 

 at the point of first 

 emergence, and de- 

 creases outward ; but 

 the horizontal element 

 is nothing at a, and in- 

 creases as the cosine of 

 the angle of emergence. 

 Therefore, under the in- 

 fluence of these two 

 conditions, one decreas- 

 ing the whole oscillation, the other increasing the horizontal element of 

 that oscillation, it is evident that there will be a point on every side, or, 

 in other words, a circle, where the horizontal element will be a maximum. 

 This is shown in Fig. 104, in which a a', ~b V , c c', etc., are the de- 

 creasing oscillation, and b ~b", c c", are the horizontal element. This 

 reaches a maximum at c. It has been found by mathematical calcula- 

 tion, based upon the supposition that the whole oscillation varies in- 

 versely as the square of the distance from X, that the horizontal ele- 

 ment will be a maximum when the angle of emergence is 54° 44'. By 

 determining by observation the circle of principal disturbance, it is 

 easy to calculate the depth a X of the focus, for it will be the apex of 

 a cone whose base is that circle, and whose apical angle is 70° 32'.* 



4. Shocks more severely felt in Mines. — It has been sometimes ob- 

 served that shocks are distinctly felt in mines which are insensible at 

 the surface. This is probably explained as follows : Let S S (Fig. 105) 

 be the surface of the ground ; and let a h represent hard, elastic strata, 

 covered with loose, inelastic materials, c c. "Now, if a series of waves 

 come in the direction of the arrows d d, and, passing through a h on 

 their way to the surface, strike upon the lower surface of c c, a portion 



104.— Diagram illustrating Circle of Principal Disturbance. 



Uallct's Report for 1858, p. 101. 



