lxXXVlii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [vol. lxxvi, 



scope of the objects for which the Society was established that it 

 threw considerable light, at any rate, on the structure of the Earth. 

 Dr. Knott had worked out a method of mathematical discussion 

 of the long-distance records of earthquakes by which it was 

 possible to calculate the form of the wave-paths along which the 

 disturbance had been propagated, without recourse to the approxi- 

 mations and assumptions previously needed, and from these 

 calculations it was possible to deduce conclusions of great interest 

 regarding the constitution and structure of the Earth. 



Mr. Caegill Gilston Knott, D.Sc, LL.D., F.R.S., then 

 proceeded to deliver 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, mathe- 

 maticians had shown that these three types of wave-motion 

 existed in and over a sphere consisting of elastic solid material. 

 Many volcanic phenomena, 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-homo- 

 geneous substance behaving like an elastic solid, with two kinds 

 of elasticity giving rise to what are called the compressional and 

 the distortional waves. The velocities of these waves are markedly 

 different, being at every depth nearly in the ratio of D8 to 1. 

 Both increase steadily within the first 1000 miles of descent 

 towards the Earth's centre, the compressional Avave-velocity rang- 

 ing from 4 - 5 miles per second at the surface to S miles per second 

 at depths of 1000 miles and more ; the corresponding velocities of 

 the distortional wave are 2 - o and 4 - 3 at the surface and at the 

 1000-mile depth respective^. At greater depths these high velo- 

 cities 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. 



