518 AIN^NUAL EEPOET SMITHSONIAjST INSTITUTION, 1915. 



Another application of the heliometer was made in his deter- 

 mination of the elements of Jupiter's satellites and of the mass 

 of Jupiter. The longitudes of the satellites can be found very 

 accurately from the usual observations of eclipses, but the lati- 

 tudes are more difficult to derive. Heliometer measures had been 

 made before by Bessel and others, but in all cases the satellite had 

 been referred to. the limb or center of the disk. Gill's method was 

 to measure the distances and position angles of the satellites rela- 

 tive to one another; for, as he had found in his observations of 

 Mars, the best results are only possible when the objects to be meas- 

 ured have no sensible disks. The observations w^ere carried out 

 in 1891. On each night the measures were reduced to a constant 

 scale by referring them to the distance between two standard stars. 

 The absolute distance between the standards was determined by a 

 lengthy compa.rison with the distances of stars employed in the 

 Victoria triangulation, whose definitive coordinates had been found 

 with an accuracy quite exceptional. These observations were the 

 beginning of a very thorough investigation of the whole problem; 

 but the further observations and the discussion of the results were 

 placed by Gill in the hands of younger men, who could give a more 

 undivided attention to the problem. The nature of the investiga- 

 tion required a repetition of the observations at a subsequent date. 

 This was made by the late Bryan Cookson at the Cape in 1901-2. 

 Photograjohic observations were made concurrently in 1891 and 

 1902, and again in 1903-4. The whole material thus collected 

 formed an exceedingly valuable source for improving the accuracy 

 of our knowledge of Jupiter's system. The detailed discussion was 

 taken up by De Sitter at Gill's suggestion; he reduced Gill's own 

 observations during a visit to the Cape (1897-99), and worked out 

 the elements and masses derivable from the whole work. It is 

 evident that Gill attached the greatest importance to this work, 

 and, though the later stages were in the charge of other workers, 

 he followed its progress to the minutest detail. His stimulating 

 influence carried it to a successful conclusion, if conclusion it can 

 be called, for in his summary of the work in the History of the 

 Cape Observatory he urges the need for an extended program of 

 future work, and appeals to astronomers to c<arry it out. His last 

 scientific effort, on the day the fatal illness began, was to write an 

 introduction to De Sitter's discussion. 



Gill's detection of the existence of magnitude equation in observa- 

 tions of right ascension with the meridian circle was an incidental 

 result of his heliometer observations at Ascension. This definitive dis- 

 covery of a systematic personality, by which faint stars are regularly 

 observed too late relatively to bright stars, has been of fundamental 

 importance in meridian work. He took great interest in the problem 



