144 



REPORT^189/ 



from such a diagram is the Greenwich mean time of the commence- 

 ment of the preliminary tremors which is near the small arrow, the 

 commencement of decided motion, and the duration of the disturb- 

 ance. After this, notes may be made of the number of maxima dis- 

 placements. 



Such notes, together with a tracing or photographic reproduction of 

 the diagram, should be sent to the Seismological Committee, British Asso- 

 ciation, Burlington House, London, W. 



In many instances the preliminary tremors, which in the illustration 

 continue over an interval of 34 minutes, may only continue over 5 or 10 

 minutes, and their duration appears, to be connected with the distance at 

 which the disturbance originated. The cause of sudden displacements 

 without preliminary tremors like those shown in fig. 3 ('British Association 

 Report,' 1896, fig. 2, p. 190) is at present unknown. They are rare, and 

 may be due to subsidence beneath the supporting pier. In a dark room, 

 and especially in a warm climate, when removing the clock-box, it is 

 quite possible that now and then a minute spider may find its way 



Fig. 5. — Pulsations at Shide. 



8.28.9.30.P.M OCT 





■.......■itn-««^««««»>mn,i,, 



Shide 



into the case. If when moving this box the boom is not set in 

 motion, the existence of the work of such an intruder may be suspected, 

 and it and its web must be removed. Such troubles are, however, 

 very rare. 



A photogram commencing with intermittent long-period movements, 

 like those shown in the upper part of fig. 4 (' British Association Report,' 

 1896, p. 200), and increasing until they resemble its lower portion, indicates 

 that the boom has been swinging from side to side under the influence of 

 air currents established inside the case. Such movements, which have 

 been called earth-tremors and microseismic storms, are at times extremely 

 regular in their character. These latter, with periods of 2 or 3 minutes, 

 are called pulsations (fig. 5. See ' British Association Report,' 1896, fig. 6, 

 p. 201). These movements are frequent during the winter months, and 

 especially at night. 



Although they form an interesting study, because they may often 

 eclipse the record of an earthquake, it is necessary that they should be 



I 



