176 REPOKT— 1895. 



depth, the resulting hodograph retains its character. Although the evi- 

 dence that there is an increase in the velocity of propagation of waves as 

 we trace thera beneath the sui'face is by no means so complete as might 

 be desii-ed (see p. 161), Dr. Schmidt compares the advantages whicli the 

 curvilinear propagation presents over that of the rectilinear transmission 

 employed by Seebach. 



It will be observed that in fig. 18 there is a great concentration of 

 earthquake rays in the epifocal region which would correspond to the 

 destruction which is so noticeable in such districts, while with rectilinear 

 radiation the absence of such concentration is not in accord with the 

 results of experience. Although both hypotheses agree iji showing a 

 higher apparent velocity near to the epicentre, in Seebach's hyperbola an 

 identical limit is reached for the apparent horizontal velocity for all 

 earthquakes, while Schmidt's modification of the laws shows that the 

 apparent velocity on the surface cannot be less than that between the 

 focus and the first coseismal, with which it varies. From this it follows 

 that for limited areas the latter method admits the possibility of very high 

 A-elocities resulting from earthquakes originating at reasonable depths. 

 With rectilinear propagation on the contrary, to obtain such high veloci- 

 ties as have been observed, it is necessary that origins should be situated 

 at enormous depths. 



Should a disturbance originate near the surface, Schmidt's hodograj^h 

 consists of two symmetrical concave branches which meet in an angle at 

 the centre, indicating that the velocity increases from the epicentre 

 outwards. 



After indicating the above and other advantages presented by the 

 hodograph over the hyperbola as representing the velocity with which 

 earth waves are apparently propagated. Dr. Schmidt takes a number of 

 earthquakes for which good time observations have been obtained and 

 plots the resulting curves. These which refer to earthquakes felt over 

 moderate areas show the characteristic inflexion point denoting an 

 increased velocity in the outer portion of the disturbed tract. 



The following are examples of his results :— 



Middle Germany, March 6, 1872. — Longest wave path 400 km. 

 Here the hodograph is distinct in character with a point of inflexion at 

 about 11 miles from its vertex, having a slope indicating a velocity of 

 2-5 miles per minute. At a distance of 36-7 miles the velocity is 15 miles 

 per minute. The curve passes much more closely through the points 

 representing time and distance than the hyperbola of Seebach. Possible 

 depth, 5 to 10 miles. Mallet's method, dependent upon a single observation, 

 gives 1-9 to 2-9 miles. 



Herzogenrath, October 22, 1873. — Longest wave path 150 km. In 

 this case the hodograph is practically concave, throughout its length 

 indicating an origin near the surface. It is indicated over a radius of 

 17 miles. Possible depth is less than 3 km. By Seebach's method it may 

 be from to 14 km. 



Swiss Earthquake, January 7, 1887. — Longest wave path, 150 km. 

 The f'eneral character of this hodograph is like the last. At the point 

 of inflexion the velocity is 170 m. per second, and at 150 km. it is 

 1,300 m. The depth of the centrum is from 1 to 6 km. 



Charleston Earthquake, August 31, 1886. — Longest wave path, 

 1,500 km. Here the hodograph is nearly a straight line. The depth 

 of the centrum may exceed 120 km. 



