3 66 POPULAR SCIENCE MONTHLY. 



sand kilometres distant, in thirteen minutes; that of October 27, 

 1894, in Santiago, Chili, in Rome, eleven thousand five hundred 

 kilometres distant, in seventeen minutes, and in Charkow, Russia, 

 two thousand kilometres from Rome, between one and two minutes 

 later. It reached Tokio at the same time, after a transit of seventeen 

 thousand four hundred kilometres. 



Still another task of modern seismology is the investigation of 

 earthquakes at sea, or seismic movements of the bottom of the ocean, 

 and the manner in which they are propagated through the water, 

 of which a very fine cartographic representation has been published 

 by Dr. C. Rudolph, of Strasburg. 



The question of the origin of earthquakes stands in constant con- 

 nection with this external development of seismology. It is signifi- 

 cant and remarkable that the answers to it, though they may be 

 given differently from different scientific points of view, are always 

 consistent in one fact, that earthquakes are a phenomenon of the 

 whole earth. Some of the investigators seek to explain them, 

 aside from those that occur in volcanic regions, as a part of the 

 great changes in the earth's crust which have taken place during 

 the last geological epoch, and are still, perhaps, taking place; 

 others find their seat and cause in the unstable condition of the 

 interior of the earth, beneath its solid and red-hot envelope. The 

 former explanation, the older and heretofore the prevalent one, is 

 called the tectonic theory, because it is based, leaving out volcanic 

 earthquakes, on the structure of the earth's crust; the second, which 

 is gaining ground, and requires no separate explanation for volcanic 

 earthquakes, may be called, reviving an expression used by L. Fr. 

 Naumann, of Leipsic, the Plutonic theory, because it goes down into 

 the unexplored depths of the earth. If seismic manifestations de- 

 pend upon the action of the whole earth, a single explanatory prin- 

 ciple, as is always the case with great natural phenomena, is not 

 sufficient, and tectonic as well as Plutonic earthquakes must be rec- 

 ognized, and the reverse. 



The tectonic theory is of geological origin, and properly sup- 

 planted the older Plutonic theory of Humboldt, which was only an 

 unverified supposition. As a whole it was first worked out by Otto 

 Volger in 1858, after various similar hypotheses had been set forth 

 by other investigators. He was confirmed by the independent re- 

 searches of Rudolf Hoernes, Edouard Suess, and most of the Ger- 

 man, French, and English seismologists. 



Their theory supposes that there are large hollow spaces in the 

 crust of the earth, into which immense falls of material take place, and 

 that these are the cause of a part of the earthquakes; that the crust 

 of the earth is often and variously disturbed in consequence of the 



