THE STRENGTH OF THE EARTH'S CRUST 507 



tain. These greater depths do not, however, so immediately 

 concern the present subject. For the outer quarter of the earth 

 both radial and transverse waves increase in velocity of trans- 

 mission with depth, showing that incompressibility and rigidity 

 increase faster than density and reach values greater than those 

 exhibited by steel at the surface of the earth. ^ 



So much is certain, but when it comes to testing the character 

 of any particular shell by means of the velocities and character of 

 the vibrations which have passed through it, there is but little 

 certainty. The difhculty of an exact interpretation is discussed 

 well by Knott.^ To illustrate the variety of opinions, •Benndorf 

 has worked out a law according to which the speed of transmission 

 increases rapidly to a depth of 200 miles (320 km.) from the sur- 

 face. Knott assumes a constancy of speed below a depth of 400 

 miles (644 km. ).^ Wiechert has concluded that there are sudden 

 changes in velocity at depths of 1200, 1650, and 2450 km. Poisson's 

 ratio which expresses the relationships of the elasticities of form 

 and volume remains, however, practically constant throughout, 

 having a mean value of 0.27.^ These changes imply surfaces of 

 discontinuity. If real, however, they are deeper than the shells 

 of the earth involved in the problems of isostasy. The conclusions 

 rest, however^ upon data of doubtful reliability. Reid has made 

 a critical examination of this subject in connection with his com- 

 prehensive study of the excellent records obtained from many 

 parts of the world of the California earthquake of 1906.^ Follow- 

 ing Wiechert's method, the curves representing the normals to 

 the wave fronts and the velocities at various depths were computed 

 from the data of the seismograms. The result showed that for the 

 radial or longitudinal wave the velocity increased rapidly with 

 depth but with decreasing rapidity, from 7 . 2 km. per second at the 



' Galitzen, Vorlesimgen iiber Seismometrie, p. 138, 1914. 



^Physics of Earthquake Phenomena (1908), chap. xii. 



^Op. cit., pp. 248-50. 



''G.W. Walker, Modem Seismology, 1913, p. 61. 



5 California Earthquake of April 18, igo6 (Report of the State Earthquake Inves- 

 tigation Commission, Vol. II, "The Mechanics of the Earthquake," by H. F. Reid). 

 Published by the Carnegie Institution of Washington, 19 10. 



