562 Dr. J. W. Evans — Presidential Address. 



by their action in reflecting vibrations from artificial explosions, 

 a procedure similar to that by means of which the presence of vessels 

 at a distance can be detected by the reflection of submarine sound 

 waves. The ordinary seismographs are not suited for this purpose; 

 the scale of their record, both of amplitude and of time, is too small 

 for the minute and rapid vibrations which would be expected to 

 reach an instrument situated several miles from an explosion, or to 

 distinguish between direct vibrations and those that may arrive 

 a second or two later after reflexion at a surface of discontinuity. 

 As the cylinder on which the record is made would be only in motion 

 while the experiment was in progress, there would be no difficulty 

 in arranging for a much more rapid movement. At the same time 

 it would be desirable to dispense with any arrangement for damping 

 the swing of the pendulum, which would be unnecessary with small 

 and rapid vibrations, and would tend to suppress them. It is 

 possible that it might be better to employ a seismograph which 

 records, like that devised by Galitzin shortly before his death, 

 variations of pressure expressing terrestrial acceleration, instead of 

 one which records directly the movements of the ground. It would, 

 however, probably be found desirable to substitute for the piezo- 

 electric record of pressure employed by Galitzin a record founded on 

 the effect of pressure in varying the resistance in an electric circuit. 

 This is, in fact, the principle of the microphone and most modern 

 telephone receivers, but quantitatively they are very unreliable. 

 This would not matter so much for the present purpose, where the 

 time of transmission is the most important feature in the evidence, 

 but satisfactory results even in this respect appear to be given by 

 Brown's liquid microphone, from which the record could be taken, 

 if desired, by means of the reflection of a mirror, attached to the 

 needle of the galvanometer. 



Evidence of the structure of the earth's crust is also afforded by 

 observations on the direction and magnitude of gravitation, which 

 have been carried out in considerable detail in India and the United 

 States — especially in the neighbourhood of great mountain ranges. 

 At the present time the problem of correlating the variations 

 observed with the underground structure is only in an embryonic 

 stage. It is probable that our greatest hope of advancing researches 

 with this object is by detailed work in areas which present no 

 marked orographical features, and where the geological structure 

 is already fairly well ascertained. 



The same remarks apply to the results obtained by magnetic 

 surveys. Apart from the marked effect of masses of magnetite in 

 the immediate neighbourhood of the surface, local magnetic 

 irregularities appear to be mainly determined by the presence of 

 basic igneous rocks, 1 but there seems to be considerable room for 

 research as to the relation between these phenomena and the form 

 and composition of an igneous intrusion. 



In this review of some of the possibilities of geological research 

 I cannot claim to have done more than touch the fringe of the 



1 Cox, Abstracts of the Proceedings of the Geological Society of London, 

 1918, pp. 71-4. 



