516 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1915. 



large volumes (vi and vii) of the Cape Annals, is a remarkable 

 record of a thorough and laborious undertaking. It was particularly 

 the kind of investigation to bring out the characteristic qualities of 

 Gill's genius. To plan the work required that perfect understand- 

 ing of instruments and observations in which he was unrivaled ; and 

 to carry it through in its completeness required a dogged persistence 

 which overcame all obstacles, an enthusiasm which shirked nothing, 

 and a power of leadership which inspired all his helpers. There 

 have been other great and successful cooperative schemes since then, 

 but we miss in them the unity of execution which the immense driv- 

 ing force of Gill's leadership supplied. 



The final result gave for the solar parallax 8''.804±0".0046, and 

 in due course this value w^as adopted (as 8".80) in the Ephemerides. 

 In so far as a single investigation can be held to settle so important 

 a constant, the solar parallax Avas now known with all the accuracy 

 required for the calculations in which it plays a part. Subsequent 

 researches have all tended to confirm Gill's value; the discordant 

 results found by other methods are disappearing, whilst the supe- 

 riority of the minor planet method has become more and more mani- 

 fest. In the Eros campaign of 1900-1901 the Cape Observatory took 

 no share, owing to the northern declination of the planet, but Gill 

 followed the investigation with keen interest and took part in the 

 arrangement of the w^ork. The results from Eros, whilst diminish- 

 ing the range of uncertainty, so far as accidental errors are con- 

 cerned, have not appreciably altered the value. Shortly before leav- 

 ing the Cape, Gill initiated a determination of the same constant by 

 means of spectroscopic observations, the line-of-sight velocity of the 

 earth relative to a star being measured at opposite seasons, so that 

 the earth's orbital velocity is found. These observations are now 

 yielding excellent results. 



We have seen that his measurements during the observations of 

 Juno at Mauritius convinced Gill of the value of the heliometer 

 as an instrument of research. In his hands it was capable of re- 

 markable accuracy. The instrument is peculiarly difficult to use, 

 and the number of those who observe with it has always been few. 

 At the time when the 4-inch instrument was constructed for him 

 the heliometer was usually regarded in England as an exercise for 

 the textbook or the examination question. Even now that its pos- 

 sibilities have been demonstrated it has not been taken up widely. 

 At the present day it is natural to prefer photographic methods, 

 which give equal or perhaps slightly superior accuracy, whilst mak- 

 ing far less demands on the observer. Perhaps, too, the prospect 

 for future progress and development is more obvious in the case 

 of photographic than of heliometer observations. Certainly Gill's 

 success with the heliometer never blinded him to the advantages 



