MODERN STUDIES OF EARTHQUAKES. 363 



of the Pacific, the Antilles, and the Mediterranean, and with places 

 also where great breaches and various disturbances are evident; that 

 they are at home likewise in volcanoes; and that they are most fre- 

 quent in the northern hemisphere, and when the earth is nearest to 

 the sun. The descriptions of powerful shocks furnish us evidence 

 of a double movement of the earth's crust — an alternate up-and- 

 down vibration and an often very marked wave motion. The de- 

 struction which earthquake shocks and waves inflict on buildings, and 

 the remarkably rapid and wide spread of the tremblings over the 

 surface of the earth, have been very diligently inquired into; and 

 when, in 1856, Naples and Calabria were visited by a great earth- 

 quake, an English investigator, Robert Mallet, made a full study of 

 it, and believed that by comparing the direction of the rents in walls 

 and buildings, which were assumed to correspond with that of the 

 tremblings, he could identify the focus of the shocks in the earth's 

 interior, and the course of the wave movement over its surface — a 

 view which has long prevailed in seismology. Still more important 

 was the work of the geologist Karl von Seebach, of Gottingen, on 

 the great earthquake in central Germany, which kept the northern 

 part of the plains of the upper Rhine, around Mayence, Grossgerau, 

 and Darmstadt, disturbed for several years after 1869. Von See- 

 bach's chief effort was to obtain the most exact data possible as to the 

 time of the beginning of the shocks from as many places as possible, 

 from which he might deduce the spot where the shocks began and 

 were strongest, the epicenter which lay directly over the point in the 

 earth's interior where the movement originated. From them he also 

 deduced a series of localities where the shocks were simultaneous and 

 of equal intensity, which could be connected by certain nearly cir- 

 cular lines called homoseists. As the distance of these from the 

 epicenter increases, the undulations take place later and are weaker, 

 and facts may be thus furnished from the velocity of propagation of 

 the shocks can be computed. The observations are also important 

 because von Seebach undertook through a simple mathematical cal- 

 culation to determine from them the situation of the forces of the 

 subterranean point where the undulations originated. 



With these investigations, the process of annihilating time and 

 space by steam and the applications of electricity was also going on. 

 By the effect of this great event, the conditions of earthquake inves- 

 tigation were revolutionized. A comparative study of the phenom- 

 ena, fundamental and essential to a science of seismology, on the 

 basis of material furnished from all the regions of the earth, was 

 rendered possible. An earthquake service was organized in Japan, 

 by J. Milne, of England; one had already been organized for a con- 

 siderable time in Italy, and the results obtained at the two places 



