F 13
supplied to the Greenwich Observatory by the Cambridge Instrument Company. A unifilar instrument for absolute determinations of Horizontal Force and Declination was received on January 24, 1927. It was constructed by Messrs. Cooke, Troughton & Simms, and is a modified form of the Indian pattern designed by Captain H.A.D. Fraser, R.E. for the Indian Government.
No aluminium was used in its construction as this metal perishes in the climate of Hong Kong. Micrometers with phosphor bronze screws and springs are fitted in place of verniers. They read to 0.1 and to 0.01 by estimation. The deflected magnet is fitted with a collimator lens at one end and a cross, ruled on optical glass, at the other end, thus eliminating Fraser's subsidiary collimator.
The vibration magnet is similarly fitted, the reading of the vertical line of the cross on each magnet being observed on a scale in the eye end of the telescope. The torsion weight for use with the deflected magnet is a zylonite disc mounted on a metal spindle. When used for the vibration magnet a metal disc of the proper weight is added, and a further weight when used in the moment of inertia determinations.
The observing telescope is capable of rotation about a horizontal axis and gives excellent definition at 24 diameters.
VIII. TIME SERVICE.
Time Ball-The Time Ball on Kowloon Signal Hill is dropped at 10 a.m. and 4 p.m. daily, except on Saturdays when it is dropped at 10 a.m. and 1 p.m., and on Sundays and Holidays when it is dropped at 10 a.m. only (120th Meridian Time).
The Ball is hoisted half mast at the 55th minute and full mast at the 57th minute. If the ball fails to drop at the correct time it is lowered at 5 minutes past the hour and the ordinary routine repeated at the following hour, if possible.
Time Signals are also given at night by means of three white lamps mounted vertically on the Observatory radio mast. The lights are extinguished momentarily every second from 8h. 55m. to 9h. 0m. p.m., except at the 28th, 29th, 54th, 55th, 56th, 57th, 58th and 59th seconds, of each minute.
The 9 p.m. signals were repeated at midnight on December 31st, the last signal indicating the close of the year 1926. The hours refer to Hong Kong Standard Time (8 hours East of Greenwich).
The Time Ball was dropped successfully 655 times. There was one failure, on October 2nd, when the line was disconnected by P.W.D. workmen before 10, thus causing the ball to fall prematurely. It was not raised at 11h. as work on the line was still in progress. The ball was not raised on July 22nd or September 27th at 10h. a.m. or 4h. p.m. owing to high wind.