RECORDING EARTHQUAKES.

New Zealand Herald, Volume LXI, Issue 18631, 12 February 1924, Page 5

RECORDING EARTHQUAKES.

PERTH'S NEW SEISMOGRAPH. A. SENSITIVE INSTRUMENT. [FROM OTTIt OWN COREE? D r>KT)ENT.] SYDNEY, Feb. 5. With the completion of the installation, of a new Milne-Shaw ; Seismograph, which r pas commenced shortly before Christmas, Perth (Western Australia) observatory now possesses the most modern earthquake recording apparatus in Australia. So sensitive is it that it has been found necessary to prepare for it new quarters, in a more seoluded and shaded position than that in which it is now placed, because even vibrations of the footsteps of visitors to the observatory affect it, and it is necessary to interpose as much shade between it and the rays of the sun as possible. The instrument, says the Government astronomer, Mr. Curlewis, responds in the most remarkable manner to the outside heating by the sun of the ground and building above, so much so that the light spot vary often . moves right off the recording sheet to left or right, resulting in a loss of the record for several hours at a time. In addition, the movements of a single person in the rooms above, and 'even of people walking along the path outside, disturb the instrument. It is impossible to ootain satisfactory records under these conditions, and the instrument will shortly uo moved to another cellar which is being Built in a secluded part of the grounds, protected by trees from the direct rays of the sun and with no high buildings over it to set 115 vibrations in the ground from wind pressure or sun action.

Under the new conditions it is hoped that the wandering of the light spot will cease and continuous registrations will result. Fortunately, when the earthqitake in Japan occurred on January 15 the light spot was on the paper, and a very good record was obtained. It served to show how very much superior the new instrn ment is to the old. The Milne-Shaw recorded - quite distinctly all the various phases preliminary, the secondary, the reflected tremors of both these and of course the surface waves and the maxiwhereas the old Milne which was erected in 1900, and which the new one repls,ceis, only registered the secondary and long waves, and these were of such slight amplitude that they would have been simply entered as " tremors " in the ledger. The instrument is a great improvement on the old Milne type in that the magnifications of thte earth tremors is 'much greater, thus enabling very faint tremors to be detected. The rhvthmical swinging of the boom— marked defect of the old typeis prevented by the introduction of effective damping, this being brought about by the action of the two pairs of strong magnets upon" a light piece of metal, attached to tho boom and resting between the magnets. Thus, if under the influence of an earthquake tremor, the boom were to be suddenly jerked away from the centre, then on the return swing the. boom is stopped by the pull of the magnets upon the piece of metal from swinging past the _ centre to any great extent.' Indeed, it is practically brought to rest immediately after the .first jolt, and is ready for the next earthquake impulse.