SEISMOLOGICAL REPORTS 47- 



be instructed to communicate with public officers or other residents 

 in the various island groups with the object of obtaining records 

 of shocks noted in the several localities. The Committee is 

 reminded that its report presented at the Adelaide meeting in 

 1893 contained the record of three very considerable elevations 

 of the land surface, namely, those that took place in the island of 

 Tanna, New Hebrides (one in 1878, and two in 1888) ; and recent 

 newspapers refer to a similar rise of the land in another portion 

 of the same group in connection with the earthquake of the 9th 

 November last. 



The suggested investigation involves several difficulties. First, 

 there is the well-known difficulty of identifying the phase of the 

 waves to which any particular record relates. For distant earth- 

 quakes it is possible to distinguish series of waves, whose velocities 

 of propagation vary considerably. But for earthquakes from 

 nearer origins, the waves overlap on the Milne seismograms of the 

 older type, made by instruments with the slow movement of the 

 recording cyhnder. It is in any case desirable that the seismo- 

 logical investigator should have a copy of the seismogram to 

 enable him where possible to compare the times for the same 

 phase of the waves ; but even so, for comparatively near earth- 

 quakes, the task of distinguishing the phases is well nigh impos- 

 sible unless the diagram is more open than is the case with the 

 records in question. I therefore trust that in all the Observatories 

 in which Milne instruments are installed the recommendation of 

 the British Association will be adopted, namely, that the instru- 

 ments; should be fitted with the quicker-moving apparatus 

 that more open diagrams may be secured. There still re- 

 mains the difficulty, which is more serious apparently in some 

 Observatories than in others, of the confusion caused by the 

 appearance of what are known as air-tremors, which fog the record 

 of the small and rapid waves of the first phase, commonly called 

 " preliminary tremors." This difficulty could be largely overcome 

 if the instrument could be fitted with a damping apparatus. But 

 the momentum of the boom of the Milne instrument when dis- 

 turbed is so small that any damping attachment would probably 

 interfei'e seriously with its sensibility. It might be considered, 

 therefore, that the time had arrived for the installation of instru- 

 ments which are as sensitive as the Milne, but which are provided 

 with smtable damping attachment. Any horizontal pendulum 

 with a heavy weight attached to the boom can be so fitted with 

 suitable damping apparatus. The British Association recom- 

 mends that where possible each Observatory should be provided 

 with a second Milne seismograph placed so as to record N-S move- 

 ments, the present instruments being generally installed to record - 

 E-W 'movements. In the opinion of their Committee it would 

 be of greater advantage to seismology if, instead of being provided 

 with another Milne instrument each Observatory had for its 

 second seismograph one of the Wiechert instruments, or else one 

 of the Mainka-Bosch or Omori-Bosch type, with provision for 



