O'Reilly — De Rossi's Seismical and Endodynamical Map. 459 



observations, tlie seismical topography of any district may be 

 traced out beforehand." Further on he says: "The want of a 

 seismical and endodynamical map of Italy soon made itself feltj 

 once the new methods of seismical study were initiated, and which 

 consist essentially in a great chronological and topographical co- 

 ordination of the history of the dynamo-geological phenomena 

 having affected our country, and in a similar co-ordination of the 

 continuous observations made of the sensible as well as insensible 

 manifestations of internal dynamic action." 



He alludes to the maps which have been frequently published, 

 showing the actions of earthquakes in certain countries and dis- 

 tricts, and states : " One thing alone constantly appears on these 

 maps, the perimeter of the surfaces shaken always presents an 

 elliptical form. My manner of understanding this elliptical form 

 of the area shaken," he says, " is that the centre of percussion 

 tends to assume a lineal form ; or, as Serpieri so well expresses it, 

 the seismical centre assumes the form of a radiant [ixidiante). 

 This radiant would consequently be the major axis of the ellipsis." 



He calls attention to Ponzi's studies " as pointing to the fact 

 that geological fractures of the earth {suolo) are the means of 

 communication of the endogenous cause of earthquakes, and act 

 as lines of interruption to the uniform propagation of the seismi- 

 cal wave." 



Wishing to verify by actual observation the^truth of this result, 

 he took occasion to map out the effects of the earthquakes which 

 affected the country near Eome in January, 1873, " wishing to 

 profit by the occasion to make an accm-ate study of the relations 

 which came to be established between the well-defined geological 

 fractures of the volcanic system of the district and the form of the 

 earthquakes having occurred. I saw that in each case it was evi- 

 dent that the fracture had acted as a seismical radiant, and that 

 by tracing out the map of the volcanic fractures I at the same 

 time traced the axes of propagation of the seismic undulations. It 

 was then that I formulated the mechanical law of earthquakes, 

 well known to all, according to which the seismical wave is, in any 

 given country, always first parallel and then normal to the axis 

 of the nearest system of geological fissuring (secondo la quale 

 Tonda seismica in ciascuna contrada e mai sempre parallela prima, 

 e poscia normale aU'asse della fenditura geologica piu vicina). 



