348 



NATURE 



[November ii, 1920 



in the task of industrial reconstruction alter 

 a devastating- war which the British scientific in- 

 strument industry, in common with other British 

 industries, has now to face and accomplish, its 

 most potent means must be the extension of scien- 

 tific research to the varied problems of the in- 

 dustry. In this work the co-operative research of 

 the association and the particular research of the 

 individual firms are essential and complementary. 

 The work of the association does not supersede, 

 but emphasises the need for, and assists, the 

 scientific research undertaken by individual 

 firms. In the same way, the pure science 

 research of the universities and kindred institu- 

 tions is essential and complementary to all re- 



search carried out by the research associations or 

 by industrial firms. The universities and their 

 like are the great sources of purely scientific re- 

 search, and to them we look for that fundamental 

 work which, probably in many cases not of im- 

 mediate applicability to industry, is bound to be 

 the foundation of future guiding principles. No- 

 where is this more fully recognised than in the 

 British Scientific Instrument Research Association. 

 Its work is also largely purely scientific, but to 

 fulfil its purpose of immediate utility to its rele- 

 vant industries it necessarily cannot always follow 

 through to completion the numerous lines of in- 

 vestigation which arise out of the problems 

 studied. 



Mi c roseisms. 



Bv J. J. Shaw. 



HOW often we use the term "terra firma"! 

 It is used despite the fact that no square 

 yard of the earth's surface is ever at rest; an 

 unending train of waves, waxing and waning in 

 amplitude, are unceasingly coursing along the 

 earth's crust and to unknown depths. The wave 

 period ranges between 4 and 8 sees. ; the ampli- 

 tude is between 1/50,000 and 1/2000 in., but 

 with a wave-length of 8 to 16 miles. The 

 speed of the waves is believed to be about 

 2 miles per sec. These microseisms have been 

 known to seismologists for twenty years or more, 

 and were originally thought to be air tremors. 

 Later, the rocking of the observatory buildings 

 in the wind was suggested as their origin, or 

 the rocking of the ground due to the motion of 

 trees in the vicinity ; but it is now established 

 that these disturbances are pure earth-movements 

 travelling over long distances. With sensitive 

 seismographs, microseisms are easily recorded, 

 but whilst hypotheses have not been lacking, their 

 origin and cause still remain unknown. 



Prof. John Milne, in 1898, suggested (" Seis- 

 mology," p. 285) that the cause may be two- 

 fold : (i) air currents and convection currents 

 within the instrument cases; {2) a ground move- 

 ment produced by rapid changes of barometric 

 load. Before that time, Bertelli and Rossi had 

 noted the connection of microseisms with baro- 

 metric change. 



In America, Burbank observed an increase in 

 amplitude when a barometric load passed from 

 land to sea, or vice versa. At the International 

 Seismological Congress held in Manchester in 

 igii funds were provided for the investigation 

 of microseisms, and Prof. O. Hecker, of Stras- 

 bourg, was deputed to undertake the work. 



Daily comparisons were made between the 

 microseisms recorded and the state of the sea 

 at Cape Grisnez, Heligoland, and Borkum, a con- 

 nection between sea waves and these movements 

 having long been suggested. The war intervened, 

 and the conclusions do not appear to have been 

 published. 



In earthquake investigation observers are by 

 NO. 2663, VOL. 106] 



this time fairly familiar with the easily recog- 

 nisable chief phases, viz. "primary" and 

 "secondary" waves, followed by "long waves" 

 which rise to a "maximum"; hence it is com- 

 paratively simple to trace any particular phase 

 around the globe, and by this means to determine 

 the respective rates of propagation and to compile 

 seismological tables for future guidance. 



Microseisms do not lend themselves so readily 

 to this kind of treatment. Fig. i illustrates a 

 section of a record when microseisms are pro- 

 nounced, and shows how similar one train of 

 waves is to the next, thus defying identification 

 of any particular wave at distant stations. 



In May, 1917, the present writer was testing 

 two similar seismographs in different buildings 

 60 ft. apart. The machines were arranged on 

 the same electrical time circuit, and both oriented 

 in the same azimuth. It was observed how 

 closely similar were the microseisms on both 

 records, showing that the air tremor hypothesis 

 was untenable. A seismograph has two kinds of 

 sensitivity — one to tilt, in which the period of the 

 pendulum plays the more important part ; the 

 other to a horizontal thrust, where the ratio of 

 the leverage, operating about the "steady-point," 

 is the chief factor. These seismographs were 

 constructed with the same sensitivity to horizontal 

 thrusts, but, 'as an experiment, the period of 

 one pendulum was raised until the sensitivity to 

 tilt was four times that of the other. Under these 

 conditions the recorded amplitude of the micro- 

 seisms was approximately the same, whereas the 

 large waves of an earthquake which were recorded 

 were from three to seven times greater on the 

 toachine more sensitive to tilt. This seems to 

 suggest that microseisms are principally a hori- 

 zontal motion ; but against this must be noted 

 that seismographs designed for vertical motion 

 record microseisms quite freely. 



The fact that microseisms could be identified on 

 instruments 60 ft. apart pointed to the advisability 

 of attacking these waves on entirely new lines, 

 viz. gradually to extend the distance between the 

 observing stations so long as the identification of 



