D;^-Nu. 27.

4

REPORT UPON THE LONGITUDE OP WELLINGTON.

should adopt in obtaining the longitude of a place situated as Wellington is, with reference to Greenwich and to our own observatory —namely, that transits of the moon's bright limb and the moon-culminating stars should be observed regularly, taking care to have the transits of the first and second limbs nearly equal each lunation ; that copies of these should be sent to Greenwich Observatory every mail, requesting in return that such corresponding transits as were obtained there might be forwarded to Wellington ; and that in the mean time a very close approximation indeed to the true longitude could be obtained by calculating the moon's E.A. from the data given in the Nautical Almanac under "Moon's Culminations;" for since the adoption, within the last few years, of Hansen's Lunar Tallies in the computation of these data, the tabular errors of the moon's place are exceedingly small, and not above one-tenth of what they were eight or nine years ago, when Burchardt's Elements were used. I do not think that comparing moon culminations with Melbourne would give a better result than taking the Nautical Almanac data. I am of opinion that a far closer determination of the difference of longitude between Melbourne and Wellington could be obtained by transmission, several times, of at least three chronometers between the two places, always provided the local time at Wellington can be obtained with precision ; the comparisons of these chronometers should be made without removing them from on board the ship they may be sent in, which can be accomplished by means of a journeyman chronometer or a time-ball. Three or four comparisons each way will, I believe, give the difference of longitude more exactly than a long series of moon culminations. I pointed out the importance of determining the latitude as well as the longitude, and would suggest the method of transits in the prime vertical with a good transit instrument, as the best where a transit circle is not available. Eor this purpose it becomes necessary to know with great precision the Polar distance of the stars observed; and as the N.A. list contains so few that will be available, our Observatory would be able to render direct assistance with its transit circle in determining the positions of the stars not in the N.A. list which may be observed. In this method it will be absolutely necessary that a very delicate level, the value of whose divisions is accurately known, should be used, as the correctness of the latitude will depend directly on the precision of the level. A very useful instrument for determining latitude, and especially differences of latitude, is the "Zenith Telescope," which is now being extensively used in India for this purpose. Enclosed is a photograph of one for India made by Messrs. Troughton and Siiiims: it is portable, and is very useful in determining latitudes in surveys; this instrument is described in Loomis' Astronomy. It appears that the transit instrument already at the disposal of the' Board has no collimation micrometer. I think it would be advisable to have one added to the instrument, and a collimation telescope procured, which should be mounted on a brick or stone pier a few feet to the North or South; the collimation error could thus be obtained with precision at any moment with facility. If it be possible to get a meridian mark at a distance not less than a mile and a half on the opposite side to the collimating telescope, it will be found very useful. In observing li. Ascensions, it is very necessary to determine the errors of collimation level and azimuth with each set of observations, and most especially with small instruments; the collimation error may be eliminated by obtaining an equal number of observations with the pivots in reversed positions ; but in moon culminations this cannot well be done, and the collimation error should be determined frequently with great precision. Mr. Woods submitted some questions relative to instituting observations of terrestrial magnetism in Wellington. lam of opinion-that it would be highly desirable to establish a system of magnetic observations at Wellington: the result from such a locality would be highly interesting, and of great scientific value. At the Melbourne Observatory, monthly determinations of the declination, dip, and horizontal force are made; and we have also a set of the Kew magnetographs (self-registering by photography), which gives a continuous record of the variations of these three forces. This, of course, involves a considerable amount of work. The monthly absolute determinations, however, with the new Kew pattern instruments, only occupy about eight hours every month, and these are the only magnetic observations taken at most Observatories where the subject of terrestrial magnetism is entertained at all. Eor these observations, a magnetic theodolite and dip circle are required—the new Kew pattern is best by far. They cost about £90 together. A small wooden building, free from iron, having two small stone piers free from the floor, is all the accommodation required for them. The self-registering magnetometers cost about £350. If magnetic observations were instituted in Wellington, it would be advisable that the observer should take a few weeks' drilling at the Melbourne Observatory. There is a set of instruments at this Observatory, of the German pattern, which is not in use now, and, if desirable, could be lent to the New Zealand Government; these instruments are much more troublesome and tedious to use than those of the Kew pattern, but quite reliable nevertheless. I believe I have now referred to all the points spoken of by Mr. Woods, and I need only add that it will afford me great pleasure to answer any further questions, and to do anything I can in assisting the Board in the objects they have in view. I have, &c, To the Board of Longitude, Eobt. L. J. Ellebt, Wellington. Government Astronomer, &c.