SUN-VIEW OF THE EARTH AT THE BEGINNING OF THE TRANSIT OF 1882.

SUN-VIEW OF THE EARTH AT THE END OF THE TRANSIT OF 1882.

one station was specially suited for mid-transit photographyCape Town. Though Natal would have been worth occupying, Cape Town was superior to every other southern station for this particular purpose. But somehow the suggestion that photographs should be secured there was overlooked, and a new cause of regret added to several which will be recognised by those who come after us as they scan the history of the late transit.

But in 1882 this method-the mid-transit photographic, method-will be the one on which, I venture to predict, chief reliance will be placed. Owing to the long duration of that transit (exceeding, by two hours, the duration of the recent transit), it will be impossible to find any pairs of stations, northern and southern, at each of which the whole transit will be favourably seen. This will be manifest from figs. 1 and 2, showing the face of the earth, turned sunwards, at the beginning and end of the transit of 1882. It will be seen that, though the dotted stations in the north will be well placed throughout the transit, there are no southern stations well placed for both the beginning and end. For to be well placed they ought to be at once near A of fig. 1, and near D of fig. 2; and these two points (owing to the long duration of the transit) are far apart on the earth's surface. The points marked 1 and 2 are those best placed in a geometrical sense; and these were indicated, eleven years ago, by the Astronomer Royal, as points one or other of which ought to be occupied by Great Britain in

But apart altogether from the difficulty of occupying these stations on the antarctic continent, they are neither of them well suited for observing both the beginning and end of transit; the sun being very low at both stations at the beginning of transit, and at one of them at the end of transit also. So far, then, as the older methods of observing transits are concerned. the transit of 1882 can only be observed by Delisle's method. But we have seen that contact observations cannot be relied upon for improving our knowledge of the sun's distance. And if they could not be relied upon for that purpose now, still less can they be relied upon in 1882, before which time astronomers will have secured valuable determinations of the sun's distance from observations of the planet Mars, during the singularly favourable opposition of 1877.*

But mid-transit can be advantageously recorded by photogenerosity, consistel in denunciations addressed to me for overlooking the point which had appeared to myself and others too obvious for special mention.

It may well be hoped that stellar photography will be employed to obtain records of the position of Mars among the stars on that occasion. This method seems to promise better results than any other yet applied, or at present available.

graphic appliances in 1882, if only suitable southern stations can be occupied for the purpose; and, as no other method is available, except the demonstrably untrustworthy Delislean method, we can scarcely doubt that an effort will be made by the scientific nations to overcome the difficulties which will certainly present themselves in the search for and occupation of stations in the southern hemisphere. The nature of these difficulties will be at once recognised from fig. 3, which shows where are the best stations of all. It will be seen that in the

northern hemisphere there will be an enormous extent of land-surface where the parallactic displacement will be great (exceeding half the maximum at stations on the northern side of the northern line, marked 5), and the solar elevation sufficient (exceeding 20° at stations within the circle marked 20°). But the corresponding southern region is, in the geographical sense, most unfavourably placed. It is indeed precisely in the heart of this region that the title is placed, in Mercator's

charts, because of the absence of any known islands in that part of the South Pacific Ocean.

And there will be this further difficulty in 1882. On the occasion of the late transit the Americans, finding that no part of the transit would be visible from their own territory, appear to have considered it their natural and obvious duty to occupy stations in Siberia, Japan, the sub-antarctic ocean, and other places, where we in England were assured that no stations would be, and that no stations could be occupied. But in 1882 the whole transit being most favourably observable from the whole of the United States, it seems not unlikely that our Transatlantic cousins will consider it their part to keep their astronomers at home, leaving to other nations the task of finding suitable southern stations. I hesitate to say that this will be their view of the matter, for it is difficult to reckon on considerations of that kind where Americans are concerned. One might have thought, that after observing the eclipse of 1869 at a hundred stations in the United States, American astronomers would have been content to leave the observations of the Mediterranean eclipse of 1870 to European astronomers. But, in point of fact, they did nothing of the kind; but, with a perversity which cannot be too strongly reprehended (at least by all who admire our laisser aller system), they insisted not only on sending over astronomers, but on positively inviting English astronomers (finding we had made no arrangements for observing the eclipse) to sail with their expedition to inaccessible Mediterranean regions. However, supposing that in 1882 the attractions of the transit, as observable at home, should prevent Americans from visiting the southern hemisphere in great strength, the duty will fall on European nations. Germany and France may then, as last December, occupy three or four southern myths. But three or four will not be enough. England will be almost bound to share in the work.

Then arises the question, Where is England to send her observers in those southern seas? Unless new islands can be discovered there, no positions worthy of her ancient fame will remain for her to occupy, save precisely those antarctic islands which were described, in 1868, by one naval authority after another, as accessible, tenable, and suitable, but, unfortunately, by the same authorities, in 1873, as inaccessible, untenable, and unsuitable. Assuming, as we may not unreasonably do, that the later description meant only that it would cost more time, trouble, and money to occupy these regions than any conceivable astronomical result could repay, we are brought back to the considerations which were urged by the Astronomer Royal as long ago as 1865, in order to bring schemes of ant

T 2

arctic explorations favourably before the notice of geographers and naval authorities. With these considerations I shall conclude; merely remarking, that whatever force his views had when presented, they have greatly increased force now that an arctic expedition is in progress, the value of the results obtained by which would be far more than doubled by a successful antarctic expedition following close upon the successful issue of that which has lately set forth. "I have learned," he wrote to the President of the Geographical Society, "through the public papers, the tenor of late discussions at the Royal Geographical Society in reference to a proposal for an expedition towards the North Pole. I gather from these that the object proposed, as bearing on science, is not so much specific as general; that there is no single point of very great importance to be obtained, but a number of co-ordinate objects whose aggregate would be valuable. And I conclude that the field is still open for another proposal, which would give opportunity for the determination of various results, corresponding in kind and importance to those of the proposed Northern Expedition, though in a different locality, and would also give information on a point of great importance to astronomy, which must be sought within a few years, and which it is desirable to obtain as early as possible. In the year 1882, on December 6, a transit of Venus over the sun's disc will occur-the most favourable of all phenomena for solution of the noble problem of determining the sun's distance from the earth, provided that proper stations for the observation can be found. (It will be remembered that it was for the same purpose that the most celebrated of all the British scientific expeditions, namely, that of Captain Cook to Otaheite in 1769, was undertaken.) For the northern stations there will be no difficulty; they will be on the Atlantic seaboard of North America, or at Bermuda; all very favourable and very accessible. For the southern stations the selection is not so easy; the observation must be made on the Antarctic Continent; if proper localities can be found there; and if the circumstances of weather, &c., are favourable, the determination will be excellent; if those favourable circumstances do not hold, no use whatever can be made of the transit. The astronomical object of a southern expedition is, I trust, sufficiently explained. In the event of such an expedition being undertaken, the precise determinations which I have indicated as bearing on the astronomical question must (from the nature of the case) take precedence of all others. But there would be no difficulty in combining with them any other inquiries, of geography, geology, hydrography, magnetism, meteorology, natural history, or any of her subject for which the localities are suitable. And I have now to request that you will have the kindness to communicate