Sessional_Paper_1907 — Page 633

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26. The observations of magnetic declination and horizontal force published in Tables XII and XIII were made with magnet No. 55 on Kew pattern unifilar magnetometer Elliot Brothers, No. 55. The dips were observed with dip-circle Dover, No. 71. The height above mean sea level at which the magnets are suspended is 116 feet and the position in which they are placed is 156 yards almost due Westward of the transit instrument. This is equivalent to a difference of longitude of 0.33. The methods adopted in making the observations and in determining and applying the corrections are explained in Appendix G of Observations and Researches made in 1885 "On the verification of unifilar magnetometer Elliot Brothers No. 55." The value of 72 K used was 3.44914 at 25° Cent. The value of P was 8.505. The mean value of the magnetic moment of the vibrating needle was 570.24. From comparisons made between magnetometers No. 55 and No. 83 in the year 1898 it was shown that the correction to the horizontal force obtained by the former as given in Tables XIII and XIV was +0.00052 (see "Observations and Researches made in 1898 "

page 19).

The times of vibration exhibited in Table XIII are each derived from 12 observations' of the time occupied by the magnet in making 100 vibrations, corrections having been applied for rate of chronometer and arc of vibration.

The observations of horizontal force given in Table XIV are expressed in C. G. S. units. The vertical and total forces have been computed by aid of the observed dips.

As in the previous year gangs of coolies were at work with pick and spade in the pear neighbourhood of the magnetic house in March and May, after which this source of possible error ceased.

27. Doctor ALESSIO of the Italian Cruiser Calabria visited the Observatory during the month of August for the purpose of making an accurate determination of the constant of gravity as well as of the magnetic elements. The magnetic hut was placed at his disposal for this work.

28. Appendix A contains 5 day means of meteorological observations from 23 years. hourly observations. This table enables one at a glance to ascertain how much the barometer, thermometer, etc., read off at any time differs from the mean value. Tracks of typhoons in 1906 are being constructed by Mr. F. G. FIGG and will be published as soon as they are ready.

Hongkong Observatory, 28th January, 1907.

W. DOBERCK,

Director.