PKESENT KNOWLEDGE OF THE EARTH WIECHERT. 441 



zeal with which earthquake investigations are conducted nowadays. 

 I must resist the temptation to present to you a general picture of this 

 activity and of its achievements, for my time is drawing to a close. 

 For the purposes of our present theme, we are concerned only with 

 one question, What do earthquake observations teach us concerning 

 the condition of the globe? 



Volcanic eruptions are accompanied by earthquakes; earthquakes 

 also result from the collapse of subterranean caves, for example, 

 those hollowed out by water. But both the " volcanic earthquakes " 

 and the " collapse earthquakes " are only of subordinate importance 

 for the globe. In the main, earthquakes occur when sudden dis- 

 placements take place in the earth's crust, along fissures either of 

 ancient origin or newly developed. The displacements thus formed 

 on the surface, and the shocks by which they are accomplished, are 

 the factors that produce the fatal effects. But all this I mention 

 merely in j^assing. The essential point for our present purpose is 

 that earth tremors starting from the focus of the quake pass through 

 the body of the globe as elastic waves. To these earthquake waves 

 we must now turn our attention. 



The oscillations of the ground caused by these waves are i^ecorded 

 by the seismometers of the seismologic stations. It is found that 

 the oscillations of great earthquakes are preceptible for hours. At the 

 same time various kinds of oscillations may be distinctly observed, 

 which pass through the globe at different rates of velocity. The 

 swiftest are those waves in which, as in the case of sound waves in 

 the air, the oscillations take place in the direction of the movement of 

 propagation to and fro, the longitudinal waves, as they are called 

 in science. Transverse waves, in which, as in the case of light, the 

 oscillations take place at right angles to the direction of propagation, 

 show only half the velocity of the longitudinal waves. Considerably 

 slower even than the transverse waves are the rocking waves, re- 

 sembling sea waves, inasmuch as they pass along the surface and do 

 not reach a great depth. It is precisely the last-arriving surface 

 waves that as a rule cause the greatest oscillations of the ground, 

 far from the focus, and which accordingly become most prominent 

 in the records of the instruments; they are on that account called 

 " principal waves." In contradistinction to them, the longitudinal 

 waves first arriving are called " first precursors " and the next-fol- 

 lowing transverse waves are called " second precursors." 



As the princi^Dal waves are limited to the surface, they can give us 

 no information on the condition of the deeper parts of the earth, and 

 hence we are not now concerned with them. The case is different 

 with the " precursors." Their behavior shows that their paths lie 

 through the body of the globe. From the observations made at the 

 surface we are, in fact, enabled by the aid of calculation to trace their 



