SIR DAVID GILL. I99 



Before the full significance of this vast undertaking, as bear- 

 ing on the cosmical problems which Gill had in view, can be 

 attained, a generation or more must elapse, when a repetition of 

 the work in whole or in part may be expected to throw light on 

 the changes that are taking place. For the investigation of such 

 changes the earlier records cannot fail to ]:)rove of the highest 

 value. 



No reference has hitherto l)een made to Gill's further re- 

 searches on the Solar Parallax. The great success of the Mars 

 expedition led him to look forward with hope to a still more ambi- 

 tious project from the favourable oppositions of Iris, Victoria and 

 Sappho in 1888-9, ^"<^ ^^ was largely with a view to this scheme 

 that the new 8-in. heliometer was installed. As, however, the 

 Observatory was not favourably situated for the employment of 

 the diurnal method and Gill's other official duties prevented an 

 extended al^sence from the Observator3% it was necessary to 

 invoke the assistance of other suitably equipped observers. ( )n 

 Gill's initiation arrangements were made which secured the co- 

 operation of the heliometers at the Cape, Yale, Leipzig, Gottingen. 

 Bamberg and Oxford while no less than 22 observatories offered 

 assistance in meridian observations of the planet and comparison 

 stars used for the heliometer observations. 



The comprehensive programme was brought to a satisfactorv 

 conclusion and an exhaustive discussion of the material made by 

 Gill, with the assistance of Elkin and Auwers. The results 

 obtained gave not only by far the most reliable determination of 

 the solar parallax, but incidentally also a strong determination of 

 the mass of the moon, the constant of nutation and the mechanical 

 dlipticity of the Earth. 



Among other valuable researches with the heliometer must be 

 mentioned an extensive series of observations of Jupiter's satel- 

 lites by Gill and Finlay in 1891-2, subsequently continued by Mr. 

 Bryan Cookson in 1901-2. 



While Gill's personal share in the observational activities of 

 the establishment was largely confined to the use of the helio- 

 meter, a steady programme of meridian observations was conti- 

 nued under his directorate, and the reductions of arrears of 

 observations accumulated by his" predecessors in office were 

 brought to com|)letion. There resulted the Cajje General Cata- 

 logue of stars for the epoch 1865, based on observations made 

 imder Sir Thomas Maclear during the years 1860-1870, and 

 catalogues for the epochs 1885, 1890, 1900. The catalogues for 

 the last epoch were no less than three in number, including the 

 determination of accurate star positions of some 8,560 stars for 

 the standardisation of the astrographic plates from observations 

 between the years 1896-1899, some 2,798 zodiacal stars as refer- 

 ence points for determinations of jjlanetary motion observed 

 between 1900-1904, and t,too southern stars, of which accurate 

 modern places were required by Prof. Boss for the completion of 

 his fundamental catalogue, observed during 1905-6. 



