492 



Under section g, he discusses the facts ascertained by him, as to 

 the decay of the wave of shock in relation to superficial distance 

 from the seismic vertical. The amplitude of the wave slowly and 

 slightly increases, and its velocity decreases. The observations are 

 not sufficient to admit of certain deduction as to what function of 

 the distance the law of decay follows. The lowest velocity at nearly 

 30 miles from the seismic vertical was still about 1 1| feet per second. 



Under r, the author has discussed systematically the facts ascer- 

 tained as to the local disturbing causes producing abrupt perturba- 

 tions of the wave of shock, and shown that they are : 



1st. Retardation by great fissures or faults, or deep valleys of dis- 

 location; the effects of these, at about Muro and Bella, amounting 

 almost to sudden extinction of the wave. 



2nd. Alternate cutting off and partial extinction by parallel 

 chains of mountains, and the effects of multiplied anti- and syn- 

 clinals. 



3rd. Increment and reduplication, with or without change of 

 wave-path, by local reflection from mountain masses. 



4th. Effects of free-lying surfaces (flanks and extremities of 

 mountain ranges) and nodal points, and production of intersecting 

 secondary shocks, and sudden reductions of energy by entrance of 

 the wave to greatly increased masses. 



5th. Effects of formation (geological), of change from one to 

 another, &c. 



6th. Effects of position of towns and cities on plain and hill, rock 

 or loose material. 



Towns on steep rock eminences, such as Saponara, are shown to 

 have suffered from an extreme conjoint velocity, viz. that of the wave 

 itself, and that of the hill-top, oscillating as an elastic pendulum. 



Of all these modifying conditions, external contour of surface, and 

 more especially the forms and directions, &c. of mountain masses 

 and deep fissures, are shown to be the most efficient. The value is 

 shown of contoured maps, models, or other such means of directing 

 the mind to the true figure of the country, in seismic researches. 



In the section $, the author arranges and discusses the facts 

 observed by him, as to secondary effects produced by the earthquakes, 

 under the heads of 



1. Earth fissures and landslips. These are in directions generally 

 more or less transverse to the wave-paths, but conform to and are 



