To show how completely the application of Halley's method has been neglected in the choice of stations for English observing parties, let the following considerations be noticed:
At northern stations Venus will be seen lower down that at southern stations, so that as she transits the upper part of the sun's disc, her chord of transit is necessarily longer at northern than at southern stations. Now Russia occupies the best northern stations, as is her due, since they fall in Russian territory. At Nertchinsk, near Lake Baikal, Russia will have an observing party; and here the transit will last longer than as supposed to be seen from the earth's centre, by fully 15 1/2 minutes. For at this place the transit will begin nearly 6 minutes early, and end nearly 10 minutes late. Now, if we had only a southern station and La Haye Sainte of the scientific Waterloo; this country's duty calls her to a post so important and so difficult of tenure, that it may fair]y be described as the Hougoumount of the position.
1 For example, Puiseux left out of consideration the dimensions of Venus's disc, regarding her transit as that of her centre. He omitted also, as unimportant, the fact that mean time and apparent time are not coincident on December 8. The correction due to this cause is considerable.
where the transit began several minutes late, and ended several minutes early, we should have a transit lasting for a shorter time than as seen from the earth's centre: and then, comparing what was observed at such a station with what was observed at Nertchinsk, we should have Halley's method applied under effective and favourable conditions. But the southern stations to which England sends observing parties are Rodriguez and Christ Church (New Zealand);l and at the former station the transit begins late and ends late, while at the latter it begins early and ends early; so that at neither is there the combination of a late beginning and an early ending, required for the effective application of Halley's method.
Now there is a station-a station which this country ought unquestionably to occupy-where the transit would be even more shortened than it is lengthened at Nertchinsk. This station is an Antarctic island on which Sir James Ross landed a party in 1846, and to which he gave the name of Possession Island. It lies due south of the southernmost extremity of New Zealand, close by the rugged shore-line of Victoria Land, and within 18 degrees of the south pole. At this station the transit will begin 6 minutes late and end 11 1/2 minutes early, or be shortened altogether no less than 17 1/2 minutes. Adding to this the lengthening of the transit by 15 1/2 minutes at Nertchinsk, we obtain a difference of duration of fully 33 minutes. Nothing like this difference was available in the transit of 1769; nothing like it will be available in 1882. I do not know the circumstances of the transits of 2004 and 2012, but it is altogether unlikely that the opportunity of applying Halley's method will be so favourable during either of these transits as in 1874. Be that as it may, however, it is absolutely certain that no opportunity equal to that which will be afforded during the transit of 1874 will recur for one hundred and thirty-two years, nor has such an opportunity been ever before offered to astronomers. Absolutely the best opportunity of applying Halley's ingenious method which has ever been afforded, or will be afforded for more than a century and a quarter, is available to astronomers during the approaching transit. The duty of seizing this opportunity belongs assuredly to our country, which alone has colonial possessions close to the station in question, and which alone also has seamen still living who have actually set their foot on Possession Island.
1 There has been a change as to the station selected in New Zealand, from Auckland to Christ Church. The change is in accordance with my own suggestions, so far as the application of Delisle's method is concerned.
I must confess that when, four years ago, I indicated this opportunity, I thought that it would have been seized at once. I thought that reconnoitring expeditions would quickly have been prepared, and that by the present time complete arrangements would have been made for landing an observing party on Possession Island in due season for the required observations. It would have been a matter of complete indifference to me whether this had been done with or without acknowledgment of the source whence the suggestion had come. But assuredly I hoped that some steps would have been taken without delay to seize an opportunity so important, the loss of which could not but reflect some degree of discredit upon the science of this country.
For up to that very time-the spring of 1869-the importance of an Antarctic expedition for observing the transit of 1882 by Halley's method had been insisted upon over and over again by leading astronomical and geographical authorities. Nay, this very station, Possession Island, had been selected as the most suitable. The feasibility of reaching it and landing on it had been insisted upon. The superior meteorological chances presented by the station, as compared with other southern stations, had been dwelt on strongly. Everything promised that before long an Antarctic reconnoitring expedition would set forth to prepare the way. It was in the full height of these anticipatory inquiries that I pointed out the inexpediency of any attempts to apply Halley's method at an Antarctic station in 1882, dwelling earnestly on the fact that when the transit began at Possession Island, in 1882, the sun would be barely five degrees above the horizon, an elevation utterly unfit for exact observations. Upon this all the plans for an Antarctic expedition in 1882 were abandoned. But although this was as it should be (for the lives of our seamen are not to be endangered without the prospect of valuable results), there was no necessity for abandoning all idea of an Antarctic expedition. The schemes set afoot for observing the transit of 1882 should have been transferred to the transit of 1874. Not a single argument which had been argued in their favour was wanting in the case of the latter transit. The main argument was greatly strengthened; for the difference of duration in 1882 would only be twenty-four minutes, if Possession Island were the selected station; where as we have seen that in 1874, the corre-sponding difference will be fully thirty-three minutes. And the fatal objection to Possession Island as a station in 1882, has no existence in the case of the transit of 1874. Instead of the utterly insufficient solar elevation of five degrees just mentioned, there will be, in 1874, a solar elevation of thirty-eight and a half degrees when the transit begins, and of twenty-five degrees when the transit ends. And necessarily all the considerations which had been urged as to the importance of Antarctic expeditions, per se, and especially of the interest which would attach to the experiences of a wintering party near the south pole of the earth, remain unchanged.