O.V THE EARTHQUAKE AND VOLCAMC I'HENOMENA 01'' JAIWX. 



167 



The averages of velocities to the Italian stations and the actual veloci- 

 ties to Mineo and Catania, which may be comjjared with the first deter- 

 mination in the preceding table, clearly show that the apparent velocity 

 to the nearer stations was lower than it was to stations which were more 

 distant : — 



Dr. A. Cancani, who has devoted much attention to the velocities with 

 which earth disturbances are transmitted, is appai-ently inclined to attri- 

 bute the high velocities sometimes observed near to an epicentre — which, 

 however, is not the case with those just quoted — to the more rapid transit 

 of a longitudinal wave. He, however, adds that at such places, and even 

 at distant j^laces, the normal and transversal waves may occur together, 

 and the velocities determined on such occasions will have intermediate 

 values. In fairly homogeneous earth it has been shown that, within 

 100 feet or so from the origin of an artilicial disturbance, a normal move- 

 ment outraces the transversal disturbance, but such a separation is not 

 observed, nor should we expect it to be observed, at distant stations 

 (see pages 162 and 163). 



When Dr. Cancani quotes my opinions that ' velocities of 2 or 3 km. 

 per second refer to the propagation of a motion not unlike the swell upon 

 an' ocean' as not being contrary to his ideas, it must be clearly under- 

 stood that I do not refer such movements to the purely distortional elastic 

 waves of an isotropic solid. ' 



Amongst other observations quoted by Dr. Cancani to show that the 

 velocity with which earthquake waves are propagated is higher nearer to 

 an epicentre than at a distance, I select the four following, which have 

 reference to the Andalusian disturbance of December 25, 1894 : — 



To Lisbon, 

 „ Pare St. Maus, 

 „ Greenwich, 

 „ Wilhelmshavcn, 



distance C30 km. velocity in km. per sec. 



1.350 

 1,020 

 2,000 



4-2 

 3-2 

 3-6 



2-8 



' R. Accad. del Lined, vol. iii. 1894, p. 410 



