• tc 



It 



''''-' spot 



ion 



on 



not 



^^^^-"cdby 



*Uit c 



it 



COUi - 



,i 



f( 



■icept 

 up 



onvards 



t 



irtifroi" 



. -^ giialli *>*- 



ilD« 



frnsi 



or Poi"" -j;^ 



., to yi^^^ 



li the ''-''' 



U^^ 



•k 



,vill 



Ch. XXIX.] 



MOTION OF THE EARTHQUAKE- WAVES. 



137 



proceed in all directions as a wave of compression displacing 

 the particles of the vibrating medium for a certain space, 

 and then allowing them to recover their original position 

 usually without fracture of the rock. The wave moves in 

 the form of a series of spherical shells, sections of which are 



represented in the 



'fci 



am 



at c c% d d', &c. When 



movem 



first felt at the surface at a point immediately above A. 



most 



im 



seismic 



vertical. The vibrations will reach the 

 points 1 and V some seconds later according to the distance 



the focus A. The wave will successively 



emer 



of such points from 



reach the points 2 



at the surface of the country will take place in a series of 



concentric rings 



om 



the shock was first felt, as in fig. 121. The wave therefore, 

 or vibratory jar, although having the appearance of being 



TCI 



om 



The circles 1 1^ and 2 2^ in figs. 



120 and 121 are called coseismal 

 circles, because all points in their 

 circumference are simultaneously 

 shaken. The reader will observe 

 that all these spherical shells c and 

 c^, d and d^^ and the points of 

 emergence, 1, 2, 3, &c., relate to 



Fig. 121. 



ansmission 



b 



the earth of a single shock, and 



^^j I • f* 1 B. Seismic vertical, 



not to a series of separate waves ^^ 2, 3. coseismic points with i', 2', 

 following each other. Mr, Eobert s', respectively. 



Mallet and the late Mr. Hopkins have endeavoured to devise 

 instruments and methods of observation, by which the rate of 

 transit of the earthquake -wave, and the depth of the focus of 

 disturbance, might be measured. 



Mr. Mallet ^ has the merit of having been the first to make 

 a practical application of the rules deduced from mechanical 



^ Great Neapolitan Earthquake of 1857; in two vols. London, 1862. 



