GEOPHYSICAL METHODS OF INVESTIGATING 237 



tional and distortional waves appear, making the conditions 

 of motion very complicated and often indecipherable.^ 

 In addition, other kinds of waves appear which are con- 

 nected only with the surface, the most important being 

 the Rayleigh waves^ which are due to the combined 

 action of the longitudinal and transverse waves at the 

 bounding surfaces and their state of vibration. They 

 displace along the earth's surface with the velocity V equals 

 0.9 F' approximately. The waves transverse to the Ray- 

 leigh waves are called Love's waves. ^ They vibrate 

 particles at right angles to the direction of the propagation 

 in the upper strata.* These oscillating movements are 

 determined with their components by various means, such 

 as pendulum weights suitably suspended or set. The rela- 

 tive movement of the pendulum mass^ about its support 

 axis illustrates, usually after magnification by lever 

 systems, the relative ground movement. 



Those instruments with optical recording devices are 

 preferable, owing to the freedom from friction of contact 

 surfaces found in other types and the facility for magnifica- 

 tion. The curves show not only the relative movement 

 of the ground, support, and pendulum mass, but also the 

 wave frequencies and time factors of main and subsidiary 

 waves. Figure 158a shows a typical seismogram or record 

 of an average distance natural earthquake wherein only the 

 surface or ground types are indicated. It is now extremely 

 important to distinguish between the several kinds of waves 

 set up previous to discussing the record. When a quake is 



1 Repts. Imp. Soc. Sci., p. 66, Gottingen, Berlin, 1919. 



^Proc. Math. Soc. {London), No. 17, p. 4; Phil. Trans. Roy. Soc. (London), 

 No. 203, p. 1, 1904. 



^"Textbook of Electricity 1907: Some Problems of Geodynamics," 

 Cambridge, 1912. 



* For the strict theory see Mainka, C, "Physik der Erdbebenwellen," 

 Berlin, 1923; Davison, C, "Manual of Seismology"; Ballore, Montesstjs 

 DE, "La geologic seismologique," pp. 453-458; Amer. Seis. Soc. Bull., Vol. 

 2, p. 127, 1912; Adams, F., and E. G., Cooker, Carnegie Inst. Pub., No. 46, 

 Washington, 1906. 



^ See Keilhack, K., "Lehrbuch der Prak. Geol.," Vol. 2, Chap. I, for some 

 complete information with figures. 



