LAMBERT: CONSTITUTION OF THE) EARTH 141 



faction, characteristic of sound). The elastic solid can transmit 

 both kinds of radiation at the same time. There is also a third 

 kind of wave possible for which there is no familiar analogue. 

 It is a surface wave whose amplitude diminishes rapidly with the 

 depth, and its theor>^ was worked out by the late Lord Rayleigh 

 and by Lamb.^^ Seismologists discern in the records of their 

 seismographs the preliminary tremors, which are separable into 

 a first phase and a second phase, and the main shock. The 

 first and second phases of the preliminary tremors are confidently 

 identified by seismologists with the arrival of the longitudinal 

 and transverse waves, which travel through the earth with 

 velocities of about 12 and 6 kilometers per second, respectively. 

 If the eai th were homogeneous they would travel in straight lines 

 but since this is not the case their paths are curv^ed in a manner 

 analogous to the curv^ature of a ray of light when passing through 

 air of varying density. At surfaces of discontinuity there would 

 occur reflections and refractions more complicated than those 

 of sound and light because of the greater generality of the media. 

 The main shock, called by seismologists the long waves, is believed 

 to represent the arrival of the Rayleigh waves which travel more 

 slowly than the other waves, ^^ and, furthermore, travel around 

 the surface. 



The phenomena of the main shock are probably complicated 

 by irregular reflections and refractions,^'^ and there is more dif- 

 ference of opinion with regard to the main shock and its relation 

 to the Rayleigh waves than with regard to the preliminary 

 tremors. 



From observations at three observatories the focus, or point 

 of origin, of an earthquake can be determined,^''' and from the 

 observed time that a tremor takes to travel from the focus to the 

 point of observation, some interesting conclusions may be drawn 

 as to the rigidity of the earth. 



33 Rayleigh. Proc. London Math. Soc. 17: 4. 1885; Lamb. Phil. Trans. A. 

 203: I. 1904. 



^^ Their velocity is 0.92 times the velocity of the transverse waves, which, as has 

 been indicated, are in turn slower than the longitudinal waves. 



'5 Walker. Modem seismology (Longman's Monographs on Physics), p. 51. 1913. 



3« Walker, op. cit., Chap. VIII. 



