220 



REPORTS ON THE STATE OP SCIENCE, ETC. — 1920. 



By Second Method. 



It will be observed that the time in column two is generally about one second, 

 which is the approximate time required for a surface wave to travel two miles, 

 thus indicating that the direction of propagation was more or less constant and 

 approximately from north to south. 



On the other hand there are differences ranging between 0"7 sec. and I'l sec. 

 Remembering the method of synchronising the clocks it is possible many of the 

 irregularities are due to personal and instrumental error. To what extent they 

 indicate that the azimuth wanders round the northern semicircle it is difficult 

 to determine, but from the fact that the southern half was never indicated, it 

 would seem feasible to presume that the waves came generally from the north. 



More precise information is very desirable, and can only be obtained from 

 not less than three stations with preferably a longer base of operation, and 

 with better timing facilities. 



It is hoped, at some future date, when three machines are simultaneously 

 available and suitable quarters and observers found, to make the experiment 

 on a ten-mile triangle. 



An attempt was made to identify the microseisms recorded at Oxford with 

 those of West Bromwich (80 miles apart), but unfortunately the booms are 

 oriented 90° from each other. From some measures made by Professor Turner 

 there was a suggestion of agreement, but nothing really tangible has at present 

 been detected. 



A fruitful investigation for observatories would be to determine whether this 

 unidirectional character of microseisms is general, and whether the azimuth 

 depends upon the contour or physical features of a country. 



From the foregoing it is clear that microseisms are real travelling waves of 

 the same character as those propagated by earthquake shocks, and if a 

 seismograph fails to perceive them then it is not recording all that is passing. 



Two stations where Milne-Shaw instruments are installed, viz., Bidston and 

 Edinburgh, seem to be very liable to microseisms. Both stations are near the 

 sea, and both stand upon the crest of a hill. 



Shide was within six miles of the open sea, but did not stand upon a hill. 

 This station did not find the microseisms more prevalent than an average station. 



Oxford and West Bromwich are well removed from the sea. They record 

 microseisms as freely as Shide. It has yet to be determined whether the sea- 

 board is more liable to these movements : the evidence points to that conclusion. 



