CO129-195 - Governor Hennessy - 1881 [1-4] — Page 29

(6)

When the error of the standard sidereal clock has been computed from observations of far

(7)

I should recommend some special provision for ascertaining the latitude of the Observatory, if it

error of the lay to compare that clock with the mean-time clock, and then to calculad there t that Lieutenant-Commander GREEN, U.S.N., commanding the United States' surveying-vessel

great and

error of the latter on mean solar time, in order that it may be set right as above explained Palos, is already in these seas, engaged in the special task of determining true positions by the best If the clocks are in different rooms, and cannot be compared directly with one another,odern methods, for purposes chiefly of marine cartography. In this branch of work, which has been it is necessary to employ huck chronometers. Or else, which is simpler and more accurselected hitherto by our Admiralty, the United States' hydrographers have already made to have in the same room with the solar clock a chronometer regulated electrically by sidereal clock, and beating in unison with it.

valle progress in many parts of the world. It was Lieutenant-Commander GREEN who detected At Greenwich, there are two chronometer error of the Lisbon longitude above referred to; and I have no doubt that, when he comes here, on a desk in one of the offices, one regulated by the solar clock, the other by the sidered he will determine the latitude of some point in the Colony with considerable accuracy. He will also clock; and, as the commutating apparatus for correcting the mean solar clock is on the same

certain our longitude, as one station in a series comprising Nagasaki, Yokohama, Shanghai, Amoy, single individual, in the shortest time possible. I think, however, that, by putting our chronic has of late been accurately re-determined by telegraph from St. Petersburg), and at the onge nograph into the sume room with the mean solar clock, we may dispense with all of these with Madras. This, however, will not do away with the advisability of a repetition of the longi- devices. It will then only be necessary to place the chronograph in circuit with the siderea ende determination as suggested in the previous paragraph, especially as such repetition will involve only

desk, the whole operation of computing, comparing and correcting is done on one spot, by Hongkong, Manila, Saigon and Singapore, connected at one end with Vladivostok (the longitude of

clock, when the click of the armature or movements of the pencil on the former, taken i connexion with the beats of the mean solar clock, will furnish means for sufficiently accurat comparison. (5). A Time-Ball and apparatus, complete with electro-magnetic mechanism for dropping the ball

at the instant of noon by the mean solar clock, and sending a return signal to the Obser vatory: also the usual air-apparatus or springs for checking its fall. Provision must be made for connecting the clock and time-ball together by an underground telegraph line, the con-

a trifling outlay.

Of the above apparatus, I recommend that the transit-instrument and collimator be ordered from

of the Strand. Messrs. TROUGHTON & SIMMS; the time-ball, chronograph, clocks and chronometers from Messrs. DENT, 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

ducting wire of which should be of large gange. This cable may be laid in earthenware pipes, Peninsula. The chief feature is a flat, gravelly hill-top, roughly circular, and some 200 feet in

and the line must be efficiently guarded by "lightning-protectors."

(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.

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 an observatory site. The second eminence would, as Mr. BOWDLER has suggested, answer very well for the chief magnetic building.

(7). A Collimating telescope--or a telescope with cross-threads in the focus, but a plain glass cap of instead of an eye-pices 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 bebind it, in a receptacle in the pier, may be seen

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 side of the island. A site in many respects suitable might be chosen on the west side, in the neigh- bourhood of Pok-fú-lam, but this would have the disadvantage of being a long way from town, and at a

in the instrument like a star of the sixth magnitude. Or a gas-jet and reflector would serve somewhat inconvenient altitude. Another drawback which it would possess, and which, indeed, holds instead of a lamp. At a distance of one mile, the hole should be about 0.3 inch in diameter. good as to sites generally in the island, is that the attraction of the abrupt adjacent hill-masses would undoubtedly affect the spirit-levels of the instruments, a point not to be overlooked when choosing a site for an observatory that may hereafter attain scientific importance.

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 verså, 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

On the whole, therefore, I concur in the Surveyor-General's opinion that Mount Elgin affords the best available site for the Observatory. From that point, the Hongkong hills in the south shut out no more than 4° or 5° of sky, and Victoria Peak itself has an elevation of only about 7°.

The Time-Ball might be erected on any commanding point which fulfils the indispensable condition of being well seen from end to end of the harbour. Certain conditional rights, the exact scope of which I have not been able to ascertain, are held by the War Department over the plot of land, partly occupied by the Tsim-shá-tsui Police Station, which seems the natural place for the time-ball mast. The War Department also hold the high ground of the southeastern promontory. I understand that the Police Station will shortly be moved a little further back on the plateau which it now occupies, so as to clear the War Department limits. The best plan, in that case, will be to let the

marine cartography, can hardly be overstated, for it is certain that a very large proportion of the hitherto small building and mast for the Time-Ball be made an adjunct to the Police Station. The ball should

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 years ago to be more than two miles in error. With its longitude determined accurately by the telegraphic 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.

be a large one, eight or nine feet in diameter, and be painted with broad bands of white and bright red. Building.The building for the Observatory must contain a general office and computing room, an office for the Director, a room for the mean solar clock, chronograph, batteries, &c., and on either side a wing room, one for the transit-instrument and sidereal clock, the other for the additional

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