3-11-2. William Henry Mahoney Christie to H. Poincaré

1899, August 311A carbon copy of Christie’s letter to Poincaré is preserved in the Cambridge University Library (RGO 7/261).

Royal Observatory, Greenwich — London, S.E.

à M. Poincaré — Président du Bureau des Longitudes

My dear Sir,

I trust you will excuse my not having replied sooner to your letter of June 23, as the question required careful consideration, and we were in the midst of the discussion of the results obtained with our longitude instruments in the years 1888 to 1898.22Dans les lettres que Christie adresse à Lœwy et à Bassot, l’Astronome royal mentionne Frank Watson Dyson et le commandant Defforges. Gilbert Defforges (1852–1915) est un ancien élève de l’École polytechnique (1870), rattaché au Service géographique de l’armée. Il a participé, avec Léon Bassot, à la détermination de la différence de longitude Paris-Greenwich en 1888. L’équipe anglaise était alors formée par Lewis et Turner. Defforges a participé également à la mesure de la longitude Paris-Greenwich de 1892 (Turner 1916). I am now pushing this on as fast as we can and hope to have all the results ready for printing before the end of this year. They include the longitude Paris-Greenwich in 1888 and 1892, Montreal-Canso-Waterville-Greenwich 1892, Killorglin-Greenwich 1898, and determinations of the difference of longitude of the French & English stations at Greenwich (a few feet apart) to test the instruments in 1893 to 1897.

Now before making a fresh determination it seems to me very desirable that our longitude results should be fully discussed and published in order that we may be able to judge whether the instruments are in all respects satisfactory and that we may put in evidence the errors to which they may be liable. And it is desirable too that the French results for the longitude Paris-Greenwich should also be published in detail.

The enclosed copies of letters I wrote to M. Lœwy and to Col. Bassot may serve to explain my position, which is that we should do all that we can to trace the cause of the discrepancy, before undertaking a fresh determination, which might otherwise only result in failure.33The manuscript is accompanied by copies of two letters, carbons of which are preserved with Christie’s papers at the Cambridge University Library (RGO 7/262). The first letter (ALS 3p) is from Christie to Lœwy; it was sent from the Royal Observatory, Greenwich, on 01.12.1898: Dear Monsieur Lœwy,
Your letter on the question of a redetermination of the longitude Paris-Greenwich arrived while we were occupaied with a determination of a longitude in the west of Ireland (Killorglin, to supplement Valentia & Waterville at the Western end of European arc of parallel) and as Mr Dyson who has been engaged in the work has not yet returned from Ireland I feel some difficulty in replying definitely till I have discussed the question with him in view of his recent experience.
I am quite disposed to agree with the Council of the Paris Observatory that the redetermination, when it is undertaken should be made by the two Observatories concerned. But I think that before the actual work is begun the instruments to be used should be thoroughly tested by determining the difference of longitude of two stations a few yards apart.
This we have done with our instruments at Greenwich, with quite satisfacory results, and it was arranged with Commandant Defforges that this should also be done at Montsouris with the instrument used by him. He is now at Constantinople, I understand, and I have not heard what has be done at Montsouris. You may remember that we have made two independent determinations of Paris-Greenwich with our instruments * and that the two results provisionally reduced are practically identical (9m20s.85 in 1888 and 9m20s.84 in 1892).
The question, then, seems to me to turn on the transit instruments used, and that is the first point to be settled. The observations recently made for the longitude of Killorglin may throw further light on the matter as far as our instruments are concerned.
In the meanwhile, I should be very glad to have your views on this troumblesome question. A discrepancy between level errors determined by nadir and by striding level seems to have much to do with it.
With kind regards, Believe me – Your very sincerely – W.H.M. Christie
* different instruments in the two cases vizz: B&C in 1888 and D&E in 1893
The second letter (ALS 4p) is from Christie to Colonel Bassot, and is dated 09.02.1899: Monsieur le Colonel,
I have to apologize for the delay in replying to your letter of January 19, which was owing to Mr Hollis’ absence from the Observatory. As he has had charge of the Longitude computations I wished to talk the matter over with him before replying, and he has only just returned. I trust the delay has not caused you inconvenience.
As regards the redetermination of the Paris-Greenwich longitude, I think before the actual work is begun the instruments to be used should be thoroughly tested by determining with them the difference of longitude of the two piers at Paris and Greenwich respectively under the same conditions as for the Paris-Greenwich Longitude. This was arranged, if I remember right with Commandant Defforges, and we have carried it out as well as we could at Greenwich, having longitude determinations on 14 nights which give a result agreeing satisfactorily with the geodetic difference. This year I hope we may be able to get some further observations meanwhile Mr Hollins is reexamining the computations which had to be made at odd times in the midst of other work, and when he has completed the reexamination, I will send you the results. I should be very glad to hear what results you have arrived at on your side. As the two independent French & English longitudes in 1888 & 1892 respectively agreed remarkably, I doubt whether anything would be gained by repeating the determination with the same instruments until we have found the cause of the discrepancy, and in any case I am afraid I could not arrange to undertake the new determination this summer. We have just completed a determination of the longitude of Killorglin in Ireland in connection with the Western end of the European arc of longitude and we want to discuss this before taking up the new longitude of Paris-Greenwich.
Prof Darwin has communicated to me the wish of the Association Géodésique that the results of the longitude operations should be published, and now that we have completed them by the Killorglin longitude we are getting them ready for publication. I am not clear, however, as to the meaning of the last sentence in your letter. Our clock errors have been long ago determined and were, I believe, compared by Commandant Defforges with the French results when he was over here, so that I do not understand what is the ‘‘travail qui n’a pas été fait’’. For the 1888 longitude we sent you our definitive clock errors on 1890 May 15 and our adopted clock rates to Commandant Defforges on 1890 Oct 17.
Believe me – Yours very truly, W. H. M. Christie
I have not heard from Col. Bassot in reply to my letter of February 9, as to whether the testing of the French instruments at Paris, as arranged with Commt Defforges, has been completed and whether the results are satisfactory.

Under all the circustances I am afraid we should not have cleared up the preliminary questions sufficiently to allow of a redetermination of the longitude being made in 1900 with all the precautions suggested by past experience and I would suggest that we should fix the year 1901 for the new determination.

Believe me, My dear Sir, Yours very truly,

W. H. M. Christie

ALS 4p. Observatoire de Paris, X5 C6, Bureau des longitudes, manuscrits mis en depot a l’Observatoire de Paris en 1985, 1987 et 1989.

Time-stamp: "27.01.2016 00:07"