Sir David Gill. 
217 
back as 1879 Professor J. C. Adams urged him to secure observations 
which would lead to a more perfect knowledge of these values than they 
were then in possession of. It was not, however, till 1891 that Gill found 
himself free enough to take up the work. When he did he adopted a 
rather singular method of securing the necessary data. Instead of 
measuring the distance from, and relative angle of the moons to their 
primary, he determined the co-ordinates of the satellites relative to one 
another. This method had only once before been adopted, by H. Struve 
in his classical investigations on the satellites of Saturn, and then with 
great success. The 1891 observations were mainly by Gill : his first 
assistant, Finlay, contributed about one-fourth of the measures 
In 1901 and 1902 similar determinations were made by Bryan Cookson 
under the direct supervision of Gill. To guard against systematic error 
in the heliometer readings — the instrument with which all the observa- 
tions were made — vitiating the final results, in 1902 a series of photo- 
graphic exposures of Jupiter and his satellites were taken. 
The value of Jupiter's mass which emerged from a consideration of all 
the observations was 
J : S :: 1 : 1047-40, 
a value almost identical with that found by Newcomb from a discussion 
of all available data. 
Another investigation which was crowded in into comparatively empty 
hours was a determination of the distance of the moon. Earlier South 
African observers had approached the problem. Lacaille found a parallax 
of 3424-6"; Henderson, 3422-5"; Stone, 3422-7". Gill's contribution to 
the investigation was to state clearly the relation which existed between 
the interdependent values— the ellipticity of the earth ; the mass of the 
moon ; the moon's mean motion — and to evaluate constants for the future 
securing of the osculating quantities. And in this we have a distinctive 
exhibition of his width of mental view. He never considered a problem 
from some limiting outlook : his view of it was always comprehensive, 
and as far as human judgment could go his consideration of it was usually 
final. 
Among other investigations which interested him was the determina- 
tion of the aberration constant, that is the ratio of the velocity of the 
earth in its orbit to the velocity of light. The lack of agreement between 
two distinct classes*of determinations worried him considerably. 
From 1892 to 1894 he instituted a series of observations with the 
Zenith telescope which gave as a value of the aberration constant, 
20-47". 
Gill's mind was peculiarly receptive to new impressions and new ideas. 
Thus when twenty to thirty years ago the labours of Huggins, Vogel, 
