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Venus in 1874 and 1882. He pressed on the attention of astronomers the importance of observing Mars; in fact, he summed up strongly in favour of the Mars method, but at the same time he urged that the future astronomical public would not be satisfied unless all practical use were made of the transits of Venus.
It is easy to criticise after events, but looking back, with our present knowledge as to where success lies, we cannot fail to be struck with the statesmanship which Airy showed in looking far ahead and calmly reviewing beforehand the broad chances of success in attempts to utilise special opportunities for the settlement of astronomical problems.
Airy's summary reads as if he had been truly a seer of what was to be, but also as if he hardly had the courage to let so uncommon an occurrence as a transit of Venus pass by without making observations. His view was that Mars, approaching within 0'4 unit of the Earth in favourable opposition, would give better values of the Sun's parallax than Venus.
Galle of Breslau proposed in 1872 that instead of Mars one of the minor planets would be more suitable, on the ground that the smaller disc of such a planet would more than make up for the greater distance from the Earth. Galle favoured observations in both hemispheres, Airy seems rather to have thought of observations to be made at a single observatory.
It was Gill's fortune to have learned at Dunecht Observatory to appreciate the worth of the heliometer as an instrument of precision; he saw that to apply the heliometer to observations for solar parallax was the great opening. Through many of the years in the '70's he worked at this problem. At Mauritius, whither he went in 1874 with Lord Lindsay to observe the transit of Venus, he made his heliometer measures of the minor planet Juno. At Ascension, in 1877, he used the heliometer to observe Mars, and his discussion of the observations gave results of high importance. Still Gill was unsatisfied: the size, colour, and phase of Mars made observations and discussion not perfectly satisfactory. The dream of his life would not be accomplished till he could determine the solar parallax afresh from observations of minor planets with a large heliometer.
He contributed a broad discussion of the other available methods in a series of papers communicated to The Observatory in 1877. He summed up in the sense that the minor planet method promised a completely satisfactory solution of the problem. This conviction was only deepened by the results of the heliometer observations of stellar parallax to which I have already referred.
Looking forward to available opportunities, Gill found that Iris in 1888 and Victoria and Sappho in 1889 would be exceptionally favourably placed; and being assured of the co-operation of Elkin at Yale, the work was undertaken. I have already alluded to the prompt response of the Admiralty to Gill's request for a 7-inch heliometer for the execution of the project.
The employment of the diurnal method at the Cape was not
completely satisfactory, nor was the transport of the observer and instrument to some equatorial station compatible with Gill's other duties. The only course open was to combine the southern heliometer observations with corresponding observations in the northern hemisphere. Hence the co-operation of the directors of the observatories that possessed heliometers-viz. Yale, Leipzig, Göttingen, Bamberg, and Oxford (Radcliffe)-was invited and promptly given.
Methods were devised to meet the case of the employment of heliometers in different hemispheres. To this end the comparison stars were as far as possible selected so that when the planet was situated at the greatest zenith distance at which good observations can be made (that is, when the parallax factor is the greatest attainable), the two comparison stars, should be situated one above and one below the planet. In this way the determination of the parallax is made to rest upon differences of two nearly equal distances measured by means of the heliometer.
Special attention was directed to the important point of arranging matters so that the observations themselves furnish the material for the determination and elimination of the errors. The method was to be, so far as practicable, "self-correcting." Errors of scale were eliminated by selecting stars nearly equidistant from the planet; errors due to personal habit of the observer were minimised by the use of a reversing prism inserted between the eyepiece and the observer's eye, so as to bring the apparent direction of separation of the images always into the same position with reference to the vertical. The same comparison stars were used in both hemispheres, and thus errors in the adopted positions of the stars were eliminated. The cunning choice of the comparison stars along the course of the three planets is well shown in the charts given in the volumes vi. and vii. of the Annals of the Cape Observatory, which contain the full account of the observations.
For the meridian observations of the stars the co-operation of many different observatories was asked for, and Sir David Gill summed up the situation as follows::
"The wide-felt scientific need of such an undertaking may be assumed to be demonstrated by the fact that almost every astronomer who was appealed to entered with heart and soul into his allotted share of the work, and probably no similar astronomical undertaking has ever before received such universal and powerful co-operation."
Twenty-two observatories co-operated. Altogether 9620 observations of 115 stars were utilised; they were discussed and combined by Dr. Auwers, who also visited the Cape and took part in the observations of Victoria. From this able and thorough discussion of the meridian observations, coupled with the heliometer triangulations, the places of the comparison stars were derived with unusual accuracy. Dr. Auwers also reduced the 760 meridian observations of the planets and derived a value of the solar parallax, viz. 8′′·806 ±0′′·03, a value which agrees well with
that derived from the heliometer observations, though it is of much smaller weight.
The resulting heliometer determinations of the solar parallax
-three independent determinations which agree within the limits of their probable errors.
Geodesy.-At a meeting of the British Association in South Africa (Report B.A., 1905) Sir David Gill gave an account of yet another enormous piece of work which has been carried out under the influence of his far-seeing initiative. He records how he felt that one of the duties laid on him by the traditions of the observatory and the labours of his predecessors was to interest himself in the geodetic survey of the colony.
Lacaille had measured an arc of 11° northwards from Cape Town in 1752. The result showed that this southern arc indicated a figure of the Earth unsymmetrical about the equator.
Nearly a century later (1841-48) Maclear revised and extended Lacaille's arc to 4°. His result showed that the arc measured was, within narrow limits, consistent with a figure of the Earth that exhibited no difference in the northern and southern hemispheres. Lacaille's contrary deduction was found to be mainly attributable to a large local disturbance of the direction of gravity at one of the terminal stations.
Soon after his appointment as Astronomer at the Cape, Sir David Gill laid before Sir Bartle Frere, then Governor of Cape Colony, a proposal to create a system of geodetic triangles as a basis for the future accurate, survey of the colony. It was a fortunate concurrence for the colonies and for geodesy that these two men-the one with his experience of Indian administration and his knowledge of the requirements of the colony, and the other with his scientific weight and practical enthusiasm-were able to deal with this question. After many administrative difficulties had been overcome, the actual work was begun in 1883. Thus was inaugurated a survey which must be an enduring benefit to the colonies, for already troubles and litigation were beginning to arise from incompatible surveys over small areas. The survey is founded upon a complete principal triangulation laid down with scientific accuracy. This could hardly have been achieved but for the initiation of Sir David Gill and the splendid devotion of Colonel Morris, R.E. (now Sir William Morris), who was in charge of the field work, ably supported by Lieut. Laffan, R. E., and other officers.
Step by step the geodetic survey has been extended, always under the scientific direction of Sir David Gill. As astronomers we may perhaps be allowed for the moment to lose sight of the
practical advantages accruing to the colonies, and to concentrate attention on the geodetic aspect of this great work.
The surveys of Cape Colony and Natal were completed in 1896, including a rediscussion of Maclear's triangulation (Geodetic Survey of South Africa, vol. i.), and a new reduction of Bailey's survey 1859-62 (vol. ii.). Rhodesia was then begun in 1897, and the triangulation was carried from near Bulawayo (lat. 22° S.) to within 75 miles of the southern end of Tanganyika (8°40' S.).
Next, in 1902, the principal triangulation of the Transvaal and the Orange River Colony was undertaken, under the superintendence of Sir W. Morris, Gill being responsible for the initiation of the work, and acting as scientific adviser. The field work was nearly completed in 1905, and the trained parties were just about to be disbanded. Sir David Gill realised that if this occurred there would be a gap left between the Limpopo River and Bulawayo in South Rhodesia, with little likelihood of its being completed. But his energetic persistence succeeded in securing support by the intervention of Sir George Darwin, whereby the relatively small link of 120 miles in the chain was made good. Thus the triangulation is complete from the southern extremity of Africa nearly to the southern end of the German Protectorate, an arc of 25° in latitude.
It is to be hoped that the German Government will be able to continue the measurement of the arc along the eastern shore of Lake Tanganyika on German territory; all the more so because, by yet another instance of Sir David Gill's energy, the measurement of an arc of 2 to the north of the German territory is now being carried out.
The delimitation of the Anglo-German boundary between British Bechuanaland and German South-West Africa along the 20th meridian gave rise to some temporary difficulties in 1896. Sir David Gill was in England at the time, and was consulted by the Colonial Office. His knowledge of local conditions was invaluable; he received instructions to proceed to Berlin, and there he was able to arrange an agreement satisfactory to both of the Governments concerned, and the direction of the work was placed in Sir David Gill's hands. Bosman's longitude arc across Bechuanaland to the 20th meridian was connected at both east and west ends with the geodetic circuit in Cape Colony.
This great system of triangulation pervading South Africa will eventually give geodetic results relating to three considerable arcs of meridian::
First, the arc along the meridian of 19° E. longitude, with an amplitude of 12.
Second, the arc along the meridian of 26° E. longitude, with an amplitude of 8°.
Third, the great arc along the 30th meridian, with an amplitude of 25°.
As far as the reductions are completed, it would appear that there is but slight deviation from Clarke's elements of the figure of
the Earth on any of these meridians. But in the second and third there seems to be evidence that the astronomical amplitude exceeds the geodetic by a small fraction of a second of arc per degree.
From the north, preliminary operations have already been begun by Captain Lyons of the Egyptian Survey to connect Cairo with the southern triangulation.
Sir David Gill's dream will be realised when, by the junction of Cairo round the east shores of the Mediterranean with Greece, and so with Struve's arc, the North Cape in lat. 70° N. is connected by triangulation with the southernmost point of Africa, lat. 35° S., an arc of meridian 105° in length.
My task has been a difficult one. Wisely or unwisely, I have attempted to set before you the broad lines along which Sir David Gill's achievements lie. In doing so, I have had, perforce, to leave many points of interest and importance untouched, even at the risk of seeming to fail in doing justice to his work.
I hope I have been able, however inadequately, to make clear to you that the recognition of the Council has fallen upon work of extraordinary scope, not only in administrative activity, but also in investigations of high refinement and permanent value.
Sir David Gill,-In presenting this medal to you, let me say that we know that for yourself the successful achievement of your tasks is the highest reward. But we could not deny ourselves the satisfaction of recording our appreciation of the mark which your labours in a distant land have made in the advance of astronomy. Let me convey to you the hope that your well-earned leisure may be filled with continued study of the science which you love.