130 



IGNEOUS AGENCIES. 



east-and-west movement, No. 2 the north-and-south movement, and 

 No. 3 the np-and-down movement. A clock is set agoing by the ar- 

 rival of the earth wave, and afterward marks seconds on the revolving 



smoked glass disk. Fig. 108 represents a 

 portion of such record. These three rec- 

 ords may be combined so as to show the 

 actual amount and direction of the earth- 

 movement (Fig. 99, p. 117). 



The important facts recorded by this 

 instrument are : 1. The instant of trans- 

 it ; 2. The direction of transit ; 3. The 

 direction of oscillation, or angle of emer- 

 gence ; 4. The amount of oscillation. 

 From these elements (if we have several 

 seismographs scattered about the country) 

 may be calculated : 1. The velocity of 

 transit; 2. The position of the focus ; 3. 

 Perhaps the form of the focus, whether 

 point or fissure ; 4. The force of the orig- 

 inal concussion. The most important of 

 these are the position and depth of the 

 focus. 



The Determination of the Epicentrum. 

 — A good seismograph, or a number of 

 these, will give the direction of transit of the surface-wave. If in 

 this way, or even by rougher methods, we get a number of these sur- 

 face-lines of transit, by fol- 

 lowing these back we get the 

 epicentrum at their intersec- 

 tion. This is Mallet's meth- 

 od. Or if, by means of many 

 seismographs giving time of 

 transit, or even by observa- 

 tories or stations of any kind 

 with accurate clocks, we get 

 several points of simultane- 

 ous arrival of the wave, then 

 by drawing a curve through 

 these points we have a coseis- 

 mal curve. A perpendicular 

 drawn from the middle point 

 of the line joining any two of 



these points will pass through the epicentrum, and two such perpen- 

 diculars would determine its position. Fig. 109 represents coseismal 



Fig. 108.— Record of a Ewing's Seismo- 

 graph: a, east-and-west motion; b, 

 north-and-south motion; c, up-and- 

 down motion (after Sekiya). 



Coseismal Lines. 



