EARTHQUAKE PHENOMENA. 425 



rests, in which the whole adherence of the body to its support, by 

 friction or cement, may be supposed to unite, and which may be 

 called the centre of adhesion. — (See Mallet, ' 'Dynamics of Earthquakes," 

 Trans. Royal Irish Acad., 1846.) The observer who fully masters 

 these mechanical conditions of motion will see what elements he must 

 collect, so that the motion impressed on bodies thus twisted may be 

 used to calculate the velocity, &c, of the wave. All observations of 

 this class, to be of scientific value, must comprise the materials, size, 

 form, weight, sort of cement, base or foundation of the bodies dis- 

 turbed, and measurements of the amount, <fec, of disturbance, with 

 any other special conditions which occur; and these will always be 

 numerous, and demand the utmost alertness and scrutiny of the ob- 

 server. The arc and azimuth of oscillation, with weight and length of 

 chain or cord of suspended lamp set swinging by shock, often afford 

 valuable information. The length to centre of oscillation is got by 

 setting the lamp swinging, and noting the vibrations made in a min- 

 ute, knowing the latitude; also iron crosses, or lamp irons bent by 

 shock. The height, form, weight, exact section at the bend, and 

 direction of deflection, to be noted. 



Whatever difference in destructive effect may be due to formation 

 or accident, it must be borne in mind that in every shock transmitted 

 from a deep centre of impulse, and passing outwards in all directions 

 in spherical shells, there will lie a coseismal circle upon the earth's 

 surface at some determinate horizontal distance from the central 

 point vertically over the centre of impulse, in which the horizontal 

 upsetting or overturning power will be a maximum, greater, casteris 

 paribus, than at any point within or without this circle: within, be- 

 cause there the direction of shock is more vertical, and therefore less 

 calculated to overturn buildings; and without, because, though more 

 horizontal, the power of the shock has become weakened by distance of 

 transmission. This may be called the Meizoseismal Circle or Zone, 

 having the radius B c, fig. 1. It may be proved that the angle of 

 emergence for this zone of maximum overthrow is constant, and 

 makes with the horizon an angle equal to 54° 44' 9" nearly, assuming 

 the energy of shock in the normal to vary as the inverse square of 

 the distance from the origin. If, therefore, the centre of the circle, 

 or point-plumb over the origin be given, or three points can be fixed 

 by observation in the meizoseismal circle, the depth of the origin below 

 the earth's surface can be calculated by the following rule: 



"Find the mean diameter of the meizoseismal circle. Then the 

 depth of the origin or centre of impulse beneath the surface is equal 

 to the diagonal of the square whose side is equal to the radius of that 

 circle." 



If the energy in the normal be assumed to vary simply as the dis- 

 tance from the origin inversely, then the constant angle of emergence 

 for maximum overthrow is 45°, and the depth of centre of impulse is 

 equal to the radius of the meizoseismal circle. 



This gives us one method of approximating seismometrically to the 

 depth below the surface of the volcanic ' 'couche" beneath. The gen- 

 eral horizontal direction of shock (radial from a point on the surface- 



