132 IGNEOUS AGENCIES. 



deep.* Both of these earthquakes also seem to have originated in a 

 fissure. 



In 1874 a not very severe earthquake shook central Germany. It 

 has been thoroughly investigated by Seebach. The epicentrum was 

 determined with great precision by erecting perpendiculars to the bi- 

 sected chords of the coseismal curves. The focus was determined as a 

 rent through four miles of rock, the center of the rent being nine or 

 ten miles in depth, f 



The velocity of transit of the waves of the Naples earthquake was 

 860 feet per second, or between nine and ten miles per minute ; that of 

 the earthquake of middle Germany was about twenty-eight miles per 

 minute. The velocity of transit in the case of the Charleston earth- 

 quake is estimated as high as one hundred or even one hundred and 

 eighty miles per minute. 



There have been many attempts to determine the depth of earth- 

 quakes by other methods, especially by using the relative velocities of 

 the spherical and the surface waves as a means of getting the angle of 



emergence ( sec. E = — J ; but such a method is evidently valueless, 



because the velocity of the spherical wave (v) is not constant. J 



Effect of the Moon on Earthquake - Occurrence. — By an extensive 

 comparison of the times of occurrence of several thousand earthquakes 

 with the positions of the moon, Alexis Perrey has made out with some 

 probability the following laws : 1. Earthquakes are a little more fre- 

 quent when the moon is on the meridian than when she is on the 

 horizon. 2. They are a little more frequent at new and full moon 

 (syzygies) than at half-moon (quadratures). 3. They are a little more 

 frequent when the moon is nearest the earth (perigee) than when she 

 is farthest off (apogee). Now, if these laws are really true, it would 

 seem that there is a slight tendency for earthquakes to follow the law 

 of tides : for the first law gives the time of flood-tide, and the second 

 and third the times of highest flood-tide. It would seem, therefore, 

 that the attraction of the sun and moon has a perceptible effect in 

 determining the time of occurrence of earthquakes. Many geologists 

 regard these laws, if established, as conclusive proof of the general 

 fluid condition of the earth beneath a comparatively thin crust. This 

 interior liquid they suppose to be influenced by the tide-generating 

 forces of the sun and moon ; but, if this were true, the effect ought 

 to be far greater than we find it. Whatever be the interior condition 



* Science, vol. ix, p. 4S9, 188V. f Seebach, Das Mittel Deutsche Erdbeben. 



% But although it is impossible thus to find the depth of the focus directly, yet indi- 

 rectly it may be found, as Seebach has shown, by the rate of decrease of the velocity of 

 the surface-wave {y'\ The deeper the focus, the slower the rate of decrease from infinity 

 at the epicentrum. 



