THE COMLWG TRANSIT OF VENUS.
ON Friday, March 14, the Astronomer-Royal read before a meeting of the Royal Astronomical Society, a letter written by him to the Secretary of the Admiralty, " expressing his views on certain articles which had appeared in the Times and the Spec- tator in regard to the two next transits of Venus." In this letter he does us the justice to remark that " the English papers are moderate and courteous in character, though distinct in their meaning" (a most desirable feature, we would note in passing). "They are based entirely," he proceeds, "upon investigations by Mr. It. A. Proctor, published in the Monthly Notices of the Royal Astronomical Society, especially in the Notice for 1869, June 11. These investigations are illustrated by maps of great clearness and of unquestionable value, and to these mainly I shall refer in the following remarks." The Astronomer-Royal then endeavours to show not only that he has selected the best available means for observing the approaching transit, but that the con- siderations to which Mr. Proctor has pointed have been duly taken into account from the beginning. In maintenance of the latter point, he refers particularly to his communication to the Astronomical Society in 1857, some account of which will be found in the Spectator for March 1.
We must confess to some surprise at the method adopted by the Astronomer-Royal in conducting his argument, and we can readily explain the reasons for our astonishment.
Let it be remembered that the whole question is whether the
value of Halley's method for the transit of 1874 had been duly appreciated by the Astronomer-Royal. This method depends on the difference of duration of Venus's transit as seen from a northern and a southern station, and the Astronomer-Royal had said respecting it that it "fails totally" in 1874; and again, that "the observable difference of duration in 1874 will probably not be half of that in 1882." He had never questioned the abstract value of the method. On the contrary, he had repeatedly and most earnestly advocated its application in 1882, notwithstanding the serious difficulties which stood in the way, even on his own preliminary and comparatively imperfect showing.
On this Mr. Proctor bad pointed out that the method not only
does not "fail totally" in 1874, but can be applied more advan- tageously than in 1882 ; and that so far is the observable difference from being less than half that in 1882, that it exceeds the latter difference in the proportion of 7 to 6 (which is now admitted). Moreover, he showed that the conditions of solar elevation, &c., would be altogether more favourable in 1874 than in 1882; and this, again, is admitted by every one.
Now, however, that the application of Halley's method is urged
where the Astronomer-Royal had said it was inapplicable, Sir George Airy discovers all at once that it has not the merits he once so freely assigned to it. He quotes Dr. Oppolz3r, of Germany, in support of this opinion ; and we cannot but express our astonish- ment that the leading astronomer of Great Britain should require support from one who is not the leading astronomer of Germany. It appears to us possible that if Dr. Oppolzsr's views had been urged as against the Astronomer-Royal's suggestions for applying Halley's method in 1882, Sir George Airy would have found it no difficult task to dispose of his critic.
We note, moreover, that the difficulties and dangers of Antarc- tic voyaging have all at once become marvellously enhanced in the Astronomer-Royal's eyes, while its advantages—not to astronomy alone, but to geography, meteorology, terrestrial mag- netism, and other departments of science—have become corre- spondingly reduced in their apparent dimensions.
But even more surprising is the use which the Astronomer- Royal makes of Mr. Proctor's reasoning. In one of his papers, Mr. Proctor had said, "even adopting" such and such estimates (which he himself, however, was very unwilling to adopt, for they were the extreme estimates of the Astronomer-Royal's First Lieutenant, Mr. Stone), " we still deduce" such and such values.
In his letter to the Secretary of the Admiralty, the Astro- nomer-Royal (who seems singularly anxious for support from without) appeals to these admissions, in the substance of an argument commencing thus, "assuming, as Mr. Proctor appears to have assumed, and with my assent thereto." This puts the Astronomer - Royal's case on the best possible footing, and apparently (but not in reality) with Mr. Proctor's " assent thereto." Then this is followed by the adoption (as illustrative of the value of the Astronomer-Royal's method) of a station which Mr. Proctor himself was the first to point out as available (Sir George Airy had rejected it, owing to a mistake resulting from the imperfections of his maps), and which is not even now among the Astronomer-Royal's selected stations. By thus estimating the value of his own method, as supposed to be applied at Mr. Proctor's suggested station, instead of adopting the more natural course of considering the method, as he himself proposes to apply it, the Astronomer - Royal nearly suc- ceeds in bringing up his method to the same level as Halley's. Tested by comparison with Sir George Airy's stations, Halley's method remains in advance even when such readily accessible stations as Tchefoo in China (actually to be occupied by a German party), and the Crozets, or Kerguelen Land, or Mac- donald Islands, or Macquarie Island, in the sub-Antarctic Seas, are considered. But considering any Antarctic station whatever, we find Halley's method set far in advance.
All this, however, is.nothing compared with the startling state- ment that everything had been duly considered in 1857. We have hitherto forborne from dwelling too heavily upon the defects of that preliminary investigation. But we are now compelled to do so, simply in order to maintain the views we have advanced against what really amounts to a flat denial. The paper in question is accessible to every one who chooses to apply for it, either at the British Museum or at the rooms of the Royal Astronomical Society. if our statements are doubted by any reader, let him satisfy himself on the matter.
Now it is the fact that in this preliminary investigation, thus complacently referred to as full and sufficient, the time assigned for the ingress of Venus differs a full hour from the true epoch (see the Nautical Almanac for 1874, p. 434), and the time assigned for her egress is three-quarters of an hour in error. Can it pos- sibly be maintained that an inquiry based on such an inexact foundation as this is a sufficient preliminary investigation of a matter so important as a transit of Venus? Then, next, the in- quiry made in 1857 into the value of Halley's method for the transit of 1874—and never resumed, be it remembered, until it was taken up by Mr. Proctor in 1869 —culminated in the words we have quoted above,—" the observable difference of duration will probably not be half of that in 1882." Now we will compare this result of the " full consideration " claimed to have been given to the subject by the Astronomer-Royal, with the actual observable differences of duration in 1874 and 1882, not using Mr. Proctor's values or suggested stations, but those indicated in the Nautical Almanac for the earlier transit, and by the Astronomer-Royal for the later. We begin with the transit of 1882. Adopting the Astronomer-Royal's estimates for the southern station, Sabrina Land, as originally suggested by him (though its advantages are geometrical only, and cannot possibly be secured), we find that the greatest observable (or, as a matter of fact, not observable) difference of duration in 1882 will be twenty-eight minutes. Now, in the transit of 1874, we omit Nertchiusk (though it is in the Nautical Almanac list of selected stations), be- cause there is reason to fear that the Russians have been deterred from occupying it (we fear by British remissness as to southern stations). We fake Tien-tsin, and find that the Nautical Almanac assigns as the duration of the transit (meaning, as usual, from the internal contacts of Venus with the sun's edge) 3 hours 51.5 min. We take as a southern station Kerguelen Land, and the Nautical Almanac gives as the duration of the transit at this station 3 hours 25.7 min.,—which is less than the duration at Tien-tsin by 28.8 min. We submit that 28.8 cannot, after "full consideration," be described as " less than the half " of 28.
We cannot but think that it would, on the whole, accord better with the high estimation in which the Astronomer-Royal is deservedly held if, in any future communication he may have occa- sion to address to the Admiralty on this subject, be should show leas anxiety to secure support from the reasoning of Dr. Oppolzer and Mr. Proctor, and should remember more exactly than he now seems to do his own former treatment of the subject. Yet more particularly—it would be well for many reasons if be would recall something of the earnestness with which he advocated Antarctic expeditions, from the time (1857) when he said that " the astrono-
mers of the future would not be satisfied" unless such expeditions were undertaken—to the epoch of the meeting of the Royal Astro- nomical Society in December, 1868, when the opinions of Admiral Richards (Hydrographer to Admiralty), of Admiral Ommanney, and of Commander Davis (who accompanied Ross in his celebrated Antarctic voyages), were cited to show that such expeditions would involve uo difficulties which British energy and courage could not readily overcome.
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