jVA rURE 



[AufiusT I, 1907 



<X)00 miles dislance, had tailed to obtain the necessary 

 funds, but at Sir Georj^c Darwin's instance contributions 

 were obtained from this Association, from the Royal 

 Society and others, to the extent of half the estimated 

 cost ; the remaining half was met by the British South 

 Africa Company. But for Darwin's happy intervention, 

 which enabled me to secure the services of Captain Gordon 

 and his party before the Transvaal Survey Organisation 

 was entirely broken up, this serious gap in the great work 

 would probably have long remained ; for it is one thing 

 to add to an existing undertaking of the kind, it is quite 

 another to create a new organisation for a limited piece 

 of work. 



Since then Colonel (now Sir William) Morris has 

 brought to a conclusion the reductions of the geodetic 

 survey of the 'I'ransvaal and Orange River Colony, and 

 his report is now in my hands for publication. 



Dr. Rubin, under my direction, at the cost of the 

 British .South Africa Company, has carried the arc of 

 meridian northwards to S. latitude 9" 42', so that we 

 have now continuous triangulation from Cape L'Agulhas 

 to within fifty miles of the southern end of Lake Tan- 

 ganyika ; that is to say, a continuous geodetic survey 

 extending over tsventy-five degrees of latitude. 



It happens that, for the adjustment of the international 

 boundary between the British Protectorate and the Congo 

 Free State, a topographic survey is at the present moment 

 being executed northward along the 30th meridian from 

 the northern border of German East Africa. A proposal 

 on the part of the Royal Society, the Royal Geographical 

 Society, the British Association, and the Royal Astro- 

 nomical Society has been made to strengthen this work 

 by carrying a geodetic triangulation through it along the 

 30th Meridian, and thus adding z\° to the .African arc. 

 These -Societies together guarantee 1000/. towards tne cost 

 of the work, and ask for a like sum from Government to 

 complete the estimated cost. The topographic survey will 

 serve as the necessary reconnaissance. The topographic 

 work will be completed by the end of January next, and 

 the four following months offer the best season of the year 

 for geodetic operations in these regions. 



There is a staff of skilled officers and men on the spot 

 sufficient to complete the work within the period men- 

 tioned, and the Intercolonial Council of the Transvaal and 

 Orange River Colony most generously offers to lend the 

 necessary geodetic instruments. The work will have to be 

 done sooner or later, but if another expedition has to be 

 organised for the purpose the work will then cost from 

 twice to three times the present amount. One cannot 

 therefore doubt that His Majesty's Government will take 

 advantage of the present offer and opportunity to vote the 

 small sum required. This done, we cannot doubt that 

 the German Government will complete the chain along 

 the eastern side of Lake Tanganyika, which lies entirely 

 within their territory. Indeed, it is no secret that the 

 Berlin -Academy of Sciences has already prepared the 

 necessary estimates with a view to recommending action 

 on the part of its Government. 



Captain Lyons, who is at the head of the survey of 

 Tgypt, assures me that preliminary operations towards 

 carrying the arc southwards from .Alexandria have been 

 begun, and we have perfect confidence that in his energetic 

 hands the work will be prosecuted with vigour. In any 

 case the completion of the African arc will rest largely 

 in his hands. That arc, if ever my dream is realised, will 

 e.xtend from Cape L'Agulhas to Cairo, thence round the 

 eastern shore of the Mediterranean and the islands of 

 Greece, and there meet the triangulation of Greece itself, 

 the latter being already connected with Struve's great 

 arc, which terminates at the North Cape in lat. 70° N. 

 This will constitute an arc of 105° in length — the longest 

 •ire nf meridian that is measurable on the earth's surface. 



The Solar Parallax. 



Much progress has been made in the exact measure- 

 ment of the great fundamental unit of astronomv — the 

 solar parallax. 



Early in 1877 ' ventured to predict ' that we should not 

 arrive at any certainty as to the true value of the solar 



' ''The Determination of the Solar Parallax," the Obsin,alo>y, vol. i. 

 IP. j8o. 



NO. 



1970, VOL. 76] 



parallax from observations of transits of Venus, but tli ,' 

 the modern helionieter applied to the measurement - 

 angular distances between stars and the star-like ima).;< 

 of minor planets would yield results of far high' 

 precision. 



The results of the observations of the minor planets Iri- 

 Victoria, and Sappho at their favourable oppositions in 

 the years 1888 and 1889, which were made with the co- 

 operation of the chief heliometer and meridian obseiv- 

 atories, fully justified this prediction.' The Sun's distance 

 is now almost certainly known within one-thousandth part 

 of its amount. The same series of observations also 

 yielded a very trustworthy determination of the mass of the 

 -Moon. 



The more recently discovered planet Eros, which in 

 1900 approached the Earth within one-third of the mean 

 distance of the Sun, afforded a most unexpected and 

 welcome opportunity for re-determining the solar parallax 

 — an opportunity which was largely taken advantage of by 

 the principal observatories of the northern hemisphere. 

 Unfortunately the high northern declination of the planet 

 prevented its observation at the Cape and other southern 

 observatories. So far as the results have been reduced 

 and published ' they give an almost exact accordance with 

 the value of the solar parallax derived from the helio- 

 meter observations of the minor planets. Iris, Victoria, 

 and Sappho in 1888 and 1889. 



But in 1931 Kros will approach the Earth within oni-- 

 sixth part of the Sun's mean distance, and the fault will 

 rest with astronomers of that day if they do not succeed 

 in determining the solar parallax within one ten- 

 thousandlh part of its amount. 



To some of us who struggled so hard to arrive at a 

 tenth part of this accuracy under the less favourable 

 geometrical conditions that were available before the dis- 

 covery of Eros, how enviable seems the opportunity ! 



-And yet, if wc come to think of it rightly, the true 

 opportunity and the chief responsibility is ours, for tiow 

 and not twenty years hence is the time to begin our pre- 

 paration ; nom is the time to study the origin of those 

 systematic errors which undoubtedly attach to some of 

 our photographic processes; and then we ought to mn- 

 struct tekscopis specially designed for the work. These 

 telescopes should be applied to the charting of the stars 

 near the path which Eros will describe at its opposition 

 in 195 1, and the resulting star-coordinates derived from the 

 plates photographed by the different telescopes should be 

 rigorously inter-compared. Then, if all the telescopes 

 give identical results for the star-places, we can be certain 

 that they will record without systematic error the position 

 of Eros. If they do not give identical results, the sourf 

 of the errors must be traced. 



The planet will describe such a long path in the sk) 

 during the opposition of 193 1 that it is already time to 

 begin the meridian observations which are necessary to 

 determine the places of the stars that are to be used for 

 determining the constants of the plates. It is desirable, 

 therefore, that some agreement should be come to With 

 respect to selection of these reference-stars, in order that 

 all the principal meridian observatories in the world iriay 

 take part in observing them. 



I venture to suggest that a Congress of Astronomers 

 should assemble in 1908 to consider what steps should 

 be taken with reference to the important opposition of 

 Eros in 193 1. 



The Stellar Universe. 



And now to pass from consideration of the dimensions 

 of our solar system to the study of the stars, or other 

 suns, that surround us. 



To the lay mind it is difficult to convey a due appreci- 

 ation of the value and importance of star-catalogues of 

 precision. As a rule such catalogues have nothing what- 

 ever to do with discovery in the ordinary sense of the 

 word, for the existence of the stars which they contain is 

 generally well known beforehand ; and yet such catalogues 

 are, in reality, by far the most valuable assets of astro- 

 nomical research. 



If it be desired to demarcate a boundary on the Earth's 

 1 " Annals of the C'lpe Observatory," vol. vi., part vi., p. 29. 

 ■■; Monthly Notices R.A.S., Hinks vol Ixiv. p. 725; Chiistie, vol. Ixvii. 

 p. 382. 



