144 Mr. John Mihip [March 20, 



of ocean beds. "With regard to the magnitude of the latter changes 

 we have learnt much from cable engineers, who have given us many 

 instances where cables lying in parallel lines, ten or fifteen miles apart, 

 have been simultaneously interrupted, and ocean depths over con- 

 siderable areas have been increased. The depth to which these large 

 faults extend is a matter of inference. We may well imagine them as 

 passing through the whole thickness of tlie earth's crust and the dis- 

 placed block falling to give up its energy to a nucleus which we know 

 transmits undulatory movements all over our globe with uniform 

 velocity. If we take this crust to be 30 miles in thickness, then 

 with Harboe's area for the superficial disturbance, the block which 

 was disturbed at the time of the Assam earthquake would be repre- 

 sented by IJ million cubic miles. 



Following the initial impulse of a large earthquake it frequently 

 happens a few minutes later that a second severe movement is felt. 

 In Japan this is popularly spoken of as the Yuri Kaishi or the return 

 shaking. This may be a second yielding within the disturbed district, 

 but from its resemblance to the main shock it suggests an echo-like 

 reflection. If we drop a bullet into a large tub of water, waves travel 

 outwards to the sides of the tub, where they are reflected, and con- 

 verge at the centre from which they set out. With the earthquake 

 waves, the reflectimr surface may be represented by the roots of 

 mountain ranges. If these are at varying distances from the origin, 

 the reflected waves would give rise to complications at the focus. 

 The transmitting medium for these waves I take to be the more or 

 less homogeneous material which lies beneath the heterogeneous crust 

 of our world. This transmits large waves with a constant velocity. 

 In the case of the Calif ornian earthquake, which originated on fault 

 lines on the western side of that country, I should imagine the 

 reflecting surface to be the Sierras, 200 miles distant. The wave group 

 would travel to these mountains and back in about four minutes, and 

 this is approximately the time interval between the two first large 

 wave gi-oups in seismograms I have of that disturbance. After the 

 first echo or echoes, an earthquake usually dies out as a series of 

 surgings which frequently have a striking similarity to each other. 

 One explanation of these rhythmical recurring groups is that they 

 simply represent times when the movement of the ground has syn- 

 chronised with the natural period of the recording instrument. 

 Although the terminal vibrations seen on a seismogram may be 

 attributed to this cause, it does not exclude the idea that rhythmical 

 beats at an origin may result in rhythmical responses at a distance. 



Side issues of seismology are quite as instructive as the information 

 we derive from the records of earthquakes. We have already refeiTed 

 to light effects which accompany large earthquakes. This, as we have 

 seen, led up to investigations connected with micro-organisms. A long 

 series of experiments which commenced in Japan and were continued 

 in the Isle of Wight, involved a series of investigations bearing upon 

 the transpiration of plants. The fundamental object of these experi- 



