196 SIR DAVID GILL. 



based on the comparisons of observations made simultaneously, or 

 almost simultaneously, from different stations widely separated 

 on the Earth's surface. This of course necessitated the employ- 

 ment of different observers and different instruments at each 

 station. 



The quantities under investigation were so minute that there 

 always remained uncertainty as to whether the results derived 

 might not in considerable measure be attributed to some personal 

 or instrumental peculiarities, rather than to the primary phenome- 

 non sought. To obviate such uncertainty, Gill proposed to make 

 use of the so-called " diurnal method," which enabled a single 

 observer, with the same instrument throughout, to make all the 

 necessary observations. The parallax was to be derived by the 

 difference in the apparent displacement of a planet in relation to 

 the surrounding stars as derived from observations soon after 

 rising, and that derived from similar observations just before 

 setting The heliometer had already proved in Gill's hands a 

 highly efficient instrument for measuring these minute displace- 

 ments, and a preliminary attempt by means of the planet Juno at 

 Mauritius had convinced him of the ultimate efficiency of the 

 method when a more extended series of observations under more 

 favourable geometrical conditions both as regards the position of 

 the observer and that of the planet could be got. 



A favourable opportunity for a further application of the 

 method occurred on the occasion of the oiJ]iosition of Mars in 

 1877. To secure the most advantageous conditions it was neces- 

 sary to occupy a station as near as possible to the e(|uator. and the 

 Island of Ascension was selected as being convenient of access 

 and, as far as could be ascertained, likely to yield favourable 

 weather conditions. The interest of the Royal Astronomical 

 Society was readily secured, and through their means the neces- 

 sary funds were obtained to provide for an expedition. 



This expedition, of which a charming account has been 

 written by Lady Gill, proved a complete success, though almost 

 doomed to failure from the outset by a serious accident to the 

 instrument, which would have deterred a less determined observer 

 from proceeding. 



Shortly after his return to England, the post of H.M. 

 Astronomer at the Cape became vacant through the retirement 

 of Mr. Stone, and Gill was selected as his successor. He occupied 

 the position for a space of 28 years, and it was here that the 

 principal contributions to his life's work were made. On his 

 arrival he found the establishment but poorly equipped and inade- 

 (juately staffed, but by his own energy and persistent pressing of 

 its claims to official support he left it with an equii^ment that 

 would bear favourable comparison with that of any existing astro- 

 nomical observatory. 



Under Stone's directorship the establishment had been almost 

 exclusivelv devoted to meridian astronomv. To enable him tO' 



