IGNEOUS AGENCIES. 



163 



Determination of the Epicentrum and Centrum. 



By means of seismometers the direction of the earth's 

 motion may be determined. If this be taken in many 

 places, and the lines of direction be protracted, they will 

 be found to meet at some point from which all seem to 

 radiate. This is the center of the circular surface-waves 

 or epicentrum. Or, by accurate clocks in many stations, 

 the time of arrival of the shock may be recorded. If, 

 now, we draw a line through all the places where the time 

 of arrival was the same, we 

 shall have a curve which 

 represents the form of the 

 wave and the center of 

 which, a, is the epicen- 

 trum. Such lines of simul- 

 taneous arrival of shock 

 are called coseismal lines 

 (c s, Fig. 85). 



The position of the cen- 

 trum or origin is much 

 more difficult to find, but 

 has been approximately 



found for several earthquakes. The general conclusion 

 thus arrived at is that an earthquake focus (centrum) is 

 usually only six to ten miles in depth, and that the shock 

 is a jar produced by the formation of a great fissure. 



Connection of Earthquakes with Phases of the 

 Moon. By careful comparison of the times of occur- 

 rence of thousands of earthquakes, it has been shown^l. 

 That they are a little more frequent when the moon 

 is on the meridian than when on the horizon. 2. Also at 

 new and full moon than at half moons. 3. Also when 

 the moon is nearest the earth than when she is farthest 

 away. Now, these are the times of flood-tide, and of 

 high flood-tides, and of highest flood-tides. Some have 

 imagined that these facts prove the existence in the 



FIG. 85. 



