Decembeb 1, 1896.] 



KNOWLEDGE 



281 



WAVES.-XII. 



EAETHQUAKES, THE PULSE, NEKVE WAVES, 

 AND TELEPATHY. 



By Vauohan Cornish, M.Sc. 



THE earth trembles with the shock of displacements 

 which occur from time to time in those superficiil 

 parts which are termed the earth's crust. These 

 displacements are the bending, crumpUng, crack- 

 ing, or slipping of the rocks, and, occasionally, 

 volcanic outbursts or upheavals. The most general de- 

 scription of the original disturbance is a wrench : that is to 

 say, a single movement which may be analysed into two 

 components, a pull and a twist, or a shear. Most earth- 

 quakes originate at a depth which is rather great compared 

 with the depths of mines, but very small compared with 

 the diameter of the earth. The vibration which they 

 produce at the surface is generally that corresponding to 

 a wrench. The movement which goes on between the 

 origin and the surface is probably, as in other cases of 

 transmission by waves, different from the disturbances 

 where the wave is set up, and where it ends. A wrench 

 both compresses and distorts the rock, and two waves 

 appear to be set up — a wave of compression and a wave 

 of distortion — which travel with different velocities. The 

 elasticity of volume of the rock — the force with which it 

 tends to recover elastically from compression — enables the 

 solid earth to transmit a wave of longitudinal displace- 

 ment, which is similar in character to the sound-producing 

 waves which are elastically transmitted in fluids. The 

 rigidity of rock, the force with which it elastically recovers 

 its shape after distortion, enables the solid earth to trans- 

 mit a wave of transverse displacement, which may be 

 compared to the light-producing waves transmitted by the 

 ether. When these two waves break simultaneously at 

 the earth's surface, the shock (as has been said) may 

 resemble the complex disturbance which originated the 

 waves ; but if one wave travel quicker than the other, the 

 character of the surface disturbance at its commencement 

 may be simpler. In practice it is found that when the 

 origin of the earthquake is at a great distance the pre- 

 liminary tremors precede the main shock by a considerable 

 interval, which indicates that some part of the disturbance 

 travels more quickly than the rest. 



The interval which elapses between a shock at the sur- 

 face near the origin and its arrival at a point near the 

 antipodes is often so short that, according to Prof. 

 Milne, the wave cannot have had time to travel round the 

 earth by transmission through the superficial layers of 

 rock, having regard to the rate at which these are known 

 to transmit wave motion. It appears, therefore, that the 

 seismic wave can be transmitted right through the earth. 

 Delicate seismographs show an almost continual trembling 

 and quivering of the earth's surface, the tremors at any 

 point being generally due to local causes, but occasionally 

 caused by distant shocks. The surface of the earth, being 

 the boundary of the transmitting medium, experiences 

 a maximum amount of disturbance. The famihar ex- 

 periment with a row of glass marbles illustrates this 

 point. If a shock be given at one end of the row, it is 

 the marble at the opposite end of the row which moves 

 most, the intermediate marbles transmitting the shock but 

 moving scarcely at all. At the boundary of the trans- 

 mitting medium a wave hn-ahs, and the energy takes on a 

 new and violent form. Thus is the shore battered by the 

 sea, and thus is the earth heated by the breaking of the 

 ether waves sent to us by the sun. 

 The " speed of an earthquake," like the speed of 



electricity, is a term which may have several different 

 meanings. The most important speed from a practical 

 point of view is the quickness with which the shock 

 reaches successive points throughout a country exposed 

 to the visitation. This is a variable velocity which 

 depends not only upon the speed of the earthquake wave, 

 but also upon the position of the origin. If the wave 

 radiates in circles from the origin, and if the circles in 

 Fig. 1 represent the wave front at successive minutes. 



then the positions <i, b, c, d, etc., are the points on the 

 earth's surface where the shock is felt at the successive 

 minutes. It is seen at a glance that the apparent surface 

 speed of the earthquake is much greater nearly above the 

 origin, and that at a distance it tends to reach a constant 

 value which is nearly that of the true rate of the wave. 

 If, however, as is probable, increase of pressure so much 

 increases the elasticity of rock that the speed of the wave 

 is greater at greater depths, the wave front will not be 

 spherical, and the " rays " drawn from the origin at right 

 angles to the wave front will not be straight lines, but 

 wUl be curved towards the surface, as Dr. A. Schmidt has 

 pointed out. ' The effect upon the surface speed is shown 

 in Fig. 2 ; it first diminishes rather rapidly until it 

 reaches a velocity equal to that of the wave at the origin, 

 but afterwards increases gradually. The progressive 

 visitation of the localities a, /«, c, d, etc., as shown in 

 these figures, is not the travel of a surface wave but the 



arrival of an obliquely-moving breaker. The disturbance 

 of level which is produced by the breaking earth-waves 

 does, however, set up a true surface wave, the ground 

 undulating much as the surface of water will undulate 

 if a submarine mine be exploded. The surface earth 

 wave is said to be a gravitation wave : that is to say, one 

 which travels by the attraction which subsists between 

 the disturbed parts and the remainder of the globe. The 

 amplitude of the surface earth wave is very small. Seismic 

 sea waves, on the contrary, are often of terrific height. 

 In these the surface wave is often due more to ruptural 

 displacement of the sea bottom than to mere oscillatory 

 j movement. The great sea waves which traversed the 

 southern oceans during the convulsions at Erakatoa were 

 presumably due to such displacement. 



j» . -;;• ■■.■ -> 



The pulse is produced by a peculiar wave which demands 

 * See Nature, October iUli, 169S. 



