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Dear Sir,―
It will be a great favour if you will send to the above address a copy of your Earthquake Register, commencing with the first of your records, or with the entry following the last of the series relating to your Station, already published as Circular No. 1, by the British Association. This Circular was posted to your address on May 9th. An endeavor will be made to issue these Circulars every six months, the entries in each ending on June 30th and December 31st.
To facilitate comparison between the various Registers, it is desirable that they be drawn up in tabular form, the headings to seven columns being as follows:―
(1) Number. This is the number of the Earthquake in your Register.
(2) Date. Month and day.
(3) Commencement. This is the Greenwich Mean Civil Time of the first trace of motion, given in hours, minutes, and decimals of the same.
(4) Maxima. These entries refer to the times of the large movements, the principal of which will be recognized by its amplitude given in the next columns.
(5) Amplitude. This refers to the half range of motion. It should be stated in millimeters and in seconds of arc and placed opposite to the time at which it was noted.
(6) Duration. Hours and minutes.
(7) Remarks.
Please send copies of important Seismograms. It is necessary that at least one point on these (the commencement or any well marked phase) should have the exact Greenwich Mean Civil Time of its occurence indicated, and also (if the hour marks are not visible) there should be a statement of the exact number of milimetres equivalent to one hour.
On Working the Instrument.
(1) The pendulum should have a period of 15 seconds.
(2) A photogram of an artificially produced swinging of the pendulum should show that it takes fully 7 minutes to come to rest from a range of 14 m.m. (amplitude 7 m.m.). Should it come to rest more quickly than this, adjustments are required.
(3) Air tremors which may be frequent in winter, at night, and with a falling temperature, may be neutralized by a copious ventilation, or, as pointed out by Mr. N.A.F.Moos, by burning a lamp or gas jet in the room at night.
Yours faitfully,
J. MILNE,
Hon. Sec. Seismological Investigation Committee,
British Association. |