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THE HONGKONG GOVERNMENT GAZETTE, 3RD SEPTEMBER, 1881.
(6). A pair of Chronometers, one rated to sidereal and the other to mean solar time, will be re-
quired, for general use and for emergencies.
(7). A Collimating telescope—or a telescope with cross-threads in the focus, but a plain glass cap instead of an eye-piece at the eye end-should be supplied, and metal supports for it to rest in. This instrument is more convenient for collimating than the Bohnenberger eye-piece, and the distant mark cannot always be counted on for the purpose, as its apparent steadiness is liable to be affected by surface radiation.
(8). Distant meridian-mark. This may be made of a sheet of iron, say two feet by three feet, firmly attached to the face of a small brick pier at some distance either to the north or the south of the transit-instrument, painted with a fine cross, and pierced at the intersection of the cross for illumination at night. It can be provided locally. The hole should be of such a size that the light from a reflecting lamp placed behind it, in a receptacle in the pier, may be seen in the instrument like a star of the sixth magnitude. Or a gas-jet and reflector would serve instead of a lamp. At a distance of one mile, the hole should be about 0.3 inch in diameter. It is well to place the mark approximately horizontal with respect to the instrument. For an Observatory at Kau-lung, it would be convenient to fix the meridian-mark in Hongkong.
The above list includes all the apparatus that is necessary for time-determinations and the time- ball. It also comprises means for an excellent telegraphic determination of the longitude of the transit- instrument, by connexion with a fixed observatory, say that of Madras. With the cooperation of the Telegraph Company, which would pretty certainly be forthcoming, I see no reason to doubt that Madras time could be transmitted to Hongkong if an assistant with a chronometer were stationed at Singapore, to repeat the signals; and, vice versa, that Hongkong time could be transmitted to Madras. From this double process, the longitude may be determined with high precision, and at no cost except that of connect- ing the Observatory, by the nearest line of telegraph, with the Central Office in Queen's Road, and sending an assistant for a few days to Singapore. The importance of such determinations, especially as aids to marine cartography, can hardly be overstated, for it is certain that a very large proportion of the hitherto accepted longitudes, dependent on observations of the Moon or on chronometric meridian distances, are seriously incorrect. Even at a place so near to Greenwich as Lisbon, the longitude was found only two With its longitude determined accurately by the telegraphic years ago to be more than two miles in error. process, the Hongkong Observatory would furnish a fundamental meridian for these seas, and marine charts would, or ought to, be brought into conformity with its exact position as thus ascertained.
I should recommend some special provision for ascertaining the latitude of the Observatory, if it were not that Lieutenant-Commander GREEN, U.S.N., commanding the United States' surveying-vessel Palos, is already in these seas, engaged in the special task of determining true positions by the best modern methods, for purposes chiefly of marine cartography. In this branch of work, which has been neglected hitherto by our Admiralty, the United States' hydrographers have already made great and valuable progress in many parts of the world. It was Lieutenant-Commander GREEN who detected the error of the Lisbon longitude above referred to; and I have no doubt that, when he comes here, he will determine the latitude of some point in the Colony with considerable accuracy. He will also ascertain our longitude, as one station in a series comprising Nagasaki, Yokohama, Shanghai, Amoy, Hongkong, Manila, Saigon and Singapore, connected at one end with Vladivostok (the longitude of which has of late been accurately re-determined by telegraph from St. Petersburg), and at the other end with Madras. This, however, will not do away with the advisability of a repetition of the longi- tude determination as suggested in the previous paragraph, especially as such repetition will involve only a trifling outlay.
Of the above apparatus, I recommend that the transit-instrument and collimator be ordered from Messrs. TROUGHTON & SIMMS; the time-ball, chronograph, clocks and chronometers from Messrs. DENT, of the Strand. I have no exact knowledge of the cost of a time-ball, but I estimate that the rest of the equipment, as described above, should be procurable for about £500, and that a complete time-ball apparatus should not cost more than £400.
Site. I have examined the site which has been proposed, at Mount Elgin, on the Kau-lung Peninsula. The chief feature is a flat, gravelly hill-top, roughly circular, and some 200 feet in diameter. There is a smaller eminence, of corresponding height, about 400 feet to the west of it. The former of these is clearly the place for the principal building. It is at a suitable altitude (100 feet above the sea) and sufficiently roomy, commands a fair sky view, and fulfils generally the requirements of an observatory site. The second eminence would, as Mr. BOWDLER has suggested, answer very well for the chief magnetic building.
Although for a time-ball, which must be visible by all the shipping, a site at Kau-lung would seem to be preferable to any other, on account of its central position and commanding view of the entire harbour, there are obvious reasons why it might be better, if possible, to have the Observatory on Hongkong itself. Unfortunately, however, the site and neighbourhood of Victoria are not available for such a purpose, as the heights which rise so steeply behind the city shut from view a large section of southern sky, extending up to 25° of altitude. This objection applies generally to sites on the north