218 REPORTS ON THE STATE OF SCIENCE, ETC. — 1920. 



Microseisms. By J. J. Shaw. 



Microseisms appear to have been a much neglected study. A few observers 

 have counted them, measured their frequency and amplitude, and noted their 

 seasonal character, but beyond this little seems to have been done. This is all 

 the more remarkable in view of the fact that microseisms, unlike earthquakes, 

 are always more or less available for investigation. 



In 1911 the International Seismological Congress in Manchester allotted 500^. 

 for their investigation, and as a result the Central Bureau at Strasbourg tabulated 

 a number of observations, and, but for the European War, would probably 

 have reported at Petrograd in 1914. If any conclusions were arrived at they 

 do not appear to have been published. 



In the 1917 report of this Committee attention was drawn to the readiness 

 with which a microseismic wave could be identified at two adjacent stations (in 

 that case, in separate buildings 60 feet apart). 



The two machines, arranged with precisely similar constants, produced 

 identical records of the microseisms; but an interesting feature was observed, 

 that, when keeping the nominal magnifications of the two machines the same, and 

 at the same time varying the relative sensitivity to tilt of one machine to as much 

 as four times the other, the amplitude shown on the film remained the same on each 

 machine. This seems to indicate that a microseismic wave is purely horizontal 

 and compressional rather than of an undulating gravitational character. 



In the same report it was suggested that, by gradually increasing the distance 

 between the recording stations (but only so long as it was possible to identify the 

 individual waves), it might be possible to trace the origin and cause of these 

 movements. 



With this object in view two suitable stations were secured. The one was 

 the writer's household cellar at West Bromwich, the other a 'dug-out' in a 

 pit bank at Millpool Colliery situated two miles away, and kindly placed 

 at our disposal by T. Davis, Esq., of the Patent Shaft and Axletree Co., of 

 Wednesbury. 



The dug-out was a tunnel 60 feet into the mound and 15 feet below the 

 surface. It lay 17° west of north of the ' home ' station. 



The first observations were made in March and April 1919, when for a few 

 weeks two Milne-Shaw machines were available. 



It was at once seen that at stations two miles apart the records of the 

 microseismic waves were almost identical. 



The clock in use at the du,g-out was not of a sufficiently high standard to 

 obtain the precise difference in time of arrival at the respective stations. 



Several seismograms were obtained during this time and were seen to be 

 similar in every detail. 



In March and April of the present year a first-class timing clock was substi- 

 tuted, and two more machines installed with the intention of timing the 

 microseismic wave over this two-mile base line. 



The usual means of synchronising were not available, therefore the clocks 

 were adjusted as follows : — 



A watch with an excellent hourly rate was chosen and carried per motor-cycle 

 between the stations. Two observations, with 30-minute intervals, were made 

 on the home clock, two on the dug-out clock, and two more on the home clock. 

 It was estimated that on favourable occasions the two clocks were set alike 

 within one-tenth of a second. The clocks were checked once per day, and the 

 waves timed by measuring on the film from a minute eclipse to the nearest apex 

 at the extreme of an excursion. 



This first method was continued from January 31 to February 15. As differ- 

 ences of 1^ to 2 seconds were shown — being probably erroneous — an effort was 

 made during March to secure a closer comparison. 



Firstly, the clocks were checked twice per day. Secondly, as, on a closer 

 scrutiny, small fluctuations in the peripheral speed of the recording drums 

 could be detected, it was seen to be inadvisable to measure any intermediate 

 point during a minute, but to rely only upon the moment when the eclipsing 

 shutter opened or closed. 



