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XCII. Proceedings of Learned Societies. 

 GEOLOGICAL SOCIETY. 



[Continued from p. 828.) 



June 9th, 1920.— Mr. 11. D. Oldham, E.R.S., President, 

 in the Chair. 



Mr. Caegill Gilston Knott, D.Sc, LL.D., E.R.S., delivered 

 a Lecture on Earthquake Waves and the Elasticity of the 

 Earth. The Lecturer remarked that the record produced on 

 delicate seismographs of the earth-movements due to distant 

 earthquakes proves that an earthquake is the source of two types 

 of wave-motion which pass through the body of the Earth, and a 

 third type which passes round the surface of the Earth. Before 

 earthquake records were obtained, mathematicians had shown 

 that these three types of wave-motion existed in and over a 

 sphere consisting of elastic solid material. Many volcanic phe- 

 nomena, however, suggest the quite different conception of a 

 molten interior underlying the solid crust. At first statement 

 these views seem to be antagonistic : but there is no difficulty in 

 reconciling them. Whatever be the nature of the material lying 

 immediately below the accessible crust, it must become at a 

 certain depth a highly- heated fairly- homogeneous substance 

 behaving like an elastic solid, with two kinds of elasticity giving 

 rise to what are called the compressional and distortional waves. 

 The velocities of these waves are markedly different, being at 

 every depth nearly in the ratio of 1*8 to 1. Both increase 

 steadily within the first thousand miles of descent towards the 

 Earth's centre, the compressional wave-velocity ranging from 

 4*5 miles per second at the surface to 8 miles per second at 

 depths of 1000 miles and more ; the corresponding velocities of 

 the distortional wave are 2*5 and 4 - 3 at the surface and at the 

 1000-mile depth respectively. At greater depths these high 

 velocities seem to fall off slightly ; but the records fail to give us 

 clear information as to velocities at depths greater than about 

 2500 miles. Down to this depth the Earth behaves towards 

 these waves as a highly-elastic solid. The elastic constants, which 

 at first increase with depth more rapidly than the density, become 

 proportional to the density, for the velocity of propagation becomes 

 practically steady. About half-way down, however, the material 

 seems to lose its rigidity (in the elastic sense of the term), and 

 viscosity possibly takes its place, so that the distortional wave is 

 killed out. In other words, there is a nucleus of about 1600 miles 

 radius which cannot transmit distortional waves. This nucleus" 

 is enclosed by a shell of highly-elastic material transmitting 

 both compressional and distortional waves exactly like an elastic 

 solid. 



