EAR THQ UAKES. 365 



of post-cards, one of which was to be returned to him at the end of 

 each week with a record of the shocks which had been felt. " The 

 barricade of post-cards was then extended farther northward, with the 

 result of surrounding the origin of certain shocks among the mountains, 

 while others were traced to the sea-shore. By systematically pursuing 

 earthquakes, it was seen that many shocks had their origin beneath the 

 sea, . . . but it was seldom that they crossed through the mountains 

 forming the backbone of the island." * When the country had been 

 thus mapped out, it was possible to choose the most advantageous sites 

 for the observatories. 



It would carry us too far into technical matters to describe the 

 method of searching into the bowels of the earth for the actual point 

 of disturbance. It must here suffice to say that if a shock be accurately- 

 timed at various places, and if the approximately circular ring where 

 it was most severe be determined, it is possible to find with fair accu- 

 racy the spot or spots under which it originated and the depth of the 

 earthquake-center. Even without accurate time-observations, Mallet 

 was able to show that the Neapolitan shock originated between three 

 and seven miles below the surface. The Yokohama earthquake of 

 J 880 appears to have had its center at a depth of from one and a half 

 to five miles. Notwithstanding that one earthquake has been estimated 

 as originating at a depth of fifty miles, it is probable that in all cases 

 the center of shock is only a few miles below the surface. 



The vagueness as to the position which has been assigned for the 

 center of disturbance in the case of particular earthquakes probably 

 depends less on the difficulty of tracing back the vibrations to their 

 origin, than on the fact that the shocks do not usually originate in a 

 single point, but rather along a line of a mile or two in length. 



As to the way in which seismic activity is distributed geographi- 

 cally over the earth's surface, certain broad conclusions have been 

 fairly well ascertained. If a map be shaded, so as to represent the fre- 

 quency of earthquakes, we see that the shading has a tendency to fall 

 into bands or ribbons, which generally follow the steeply sloping shores 

 of continents and islands ; and it is probable that the actual origins of 

 the shocks are generally situated under the sea not far from the coast. 



It is a further interesting peculiarity that the most important bands 

 fall end to end, so that they may be regarded as a single ribbon em- 

 bracing nearly half the earth. It may be suspected that this ribbon 

 really meets itself and forms a closed curve, but this can not be proved 

 as yet. We may begin to trace its course at Cape Horn, whence it fol- 

 lows the Andes along the whole western coast of South America. At 

 the north of that continent it becomes somewhat broader, but its course 

 is clearly marked along the line of the West Indies from Trinidad to 

 Cuba. Hence it passes to the mainland in Mexico, and then runs along 

 the whole western coast of North America. We then trace the line 



* Milne, " Earthquakes," p. 189. 



