667189-1886-Astronomical-Instruments-at-Observatory-and-Time-Service-1885- — Page 1

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THE HONGKONG GOVERNMENT GAZETTE, 15TH MAY, 1886.

GOVERNMENT NOTIFICATION.-No. 177.

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The following Report of the Director of the Observatory on the Astronomical Instruments at the it isbservatory and on the Time-Service of Hongkong in 1885, is published for general information.

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Colonial Secretary's Office, Hongkong, 15th May, 1886.

FREDERICK STEWART,

Acting Colonial Secretary.

amo EPORT ON THE ASTRONOMICAL INSTRUMENTS AT THE OBSERVATORY AND -rebel

ON THE TIME-SERVICE OF HONGKONG IN 1885.

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The main purpose of the astronomical branch of the Observatory is the determination of local time, but instruments are also available for making observations of such astronomical phenomena as happen to be particularly conspicuous in this region.

Local time is determined by aid of the transit-instrument, by Troughton and Simms, of three feet -in focal length with object glass of three inches aperture. Two setting circles, read off by levels, are m fixed on the telescope near the eye-end, an arrangement very handy for observing stars passing the is meridian in quick succession. The axis is perforated for side-lamps. The pivots are made of chilled Theell-metal. There is no perceptible difference between their diameters, but minute irregularities appear theo exist, though too small to allow their exact amount to be determined by aid of the axis-level. The 7 glatter is used for obtaining the inclination of the axis, which is done to about one inch in thirty miles. A similar level to show seconds for use with the zenith micrometer, in the observation of differences of na zenith distances on either side of the zenith, is fixed and can revolve at right angles to the axis. ok The eye-piece is furnished with seven fixed vertical wires and one moveable. The latter is moved ar by aid of a screw, the value of one revolution of which is 60."62 as obtained by aid of Polaris on February 28th and April 16, 1884. The number of entire revolutions traversed by the wire is read th on a comb seen in the field of vision and the decimals are read on the head of the screw, which is a divided to hundredths, so that the space traversed is read off to within 0."06. But the moveable wire after a short time began to get entangled with the permanent wires and broke after a few months. Now although this wire materially facilitates the accurate determination of the constants of the instru- fment and the azimuth of the meridian-marks, this assistance would be too dearly bought, were the ca wires to become constantly deranged and frequently broken, so that new wire-distances &c. would have 7o be determined. This wire has therefore not been refixed.- -The distances of the six wires from the as middle wire were determined from about 19 transits of Polaris in 1884 as follows, (upper transit

clamp west):-

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20. 703, 10. 441, 5. 132 | 5% 245, 10. 250, 20o. 725

re These values may be trusted to within a hundredth of a second.-In 1884 and 1885 transits over af

seven wires were observed, but since the beginning of 1886 the transits are observed over the five the equidistant wires, which is much more convenient and just as accurate. There are three Ramsden eye-pieces, of which no use is made as high objects cannot be seen with them, but a long diagonal meye-piece is in constant use and is in fact hardly ever detached, to which circumstance the permanency sp of the seven wires may be ascribed. A bag of camphor is kept in the hollow axis of the instrument in

order to keep away insects.

There are also two horizontal wires, about a minute of arc asunder, in the middle between which is the object to be observed is placed. In order to observe differences of zenith-distance for latitude, the eye-piece can be revolved a quarter circumference round its axis, so that the vertical wires including the mmoveable wire are placed horizontal. But as the instrument is in constant use as a transit, there are so no opportunities for such work, which is moreover superfluous, as the latitude was accurately deter- as mined by Colonel PALMER in 1882.

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The telescope rests upon a cast-iron stand with reversing apparatus (essentially an excentric circle acted on by a lever). The latter is so perfect that any change of inclination caused by the reversion has never been perceived. The inclination and azimuth are adjusted by screws fixed on either side of ke the stand. The changes in these constants are probably caused mainly by expansion or contraction of tir the adjusting screws with changes of temperature. The stand rests on a portland stone slab (3 feet 56 long, 14 feet broad and 1 foot thick), which is laid in eement on top of a brick pier, sunk 5 feet deep nd in the ground, where it is surrounded by a cylinder also built in brick to protect it from surface-oscilla- top ion of the ground. The part of the pier above the floor is neatly cased in teak wood and does not mettouch the floor.

rine The constants of the instrument were determined as follows in 1884 and 1885: the error of po collimation, c, (90°c being the angle between the optical axis of the telescope and the axis of rota- ut tion on the side that carries the clamp) and the azimuth, a, (90°-a being the azimuth, counting from by youth towards west, of the westend of the axis) were determined from observations of Polaris in con- the nection with stars near the equator or on some occasions by observations of stars near the zenith and ful near the southern horizon. The inclination, b, or the altitude above the horizon of the westend of the thiraxis was obtained by aid of the level.