Sil- David Gill. 
209 
any similar undertaking, that the cliain of triangles ran right up to 
Tanganyika, thus giving a measured arc of meridian longer than on any 
other part of the earth's surface. Even had he accomplished nothing 
more than this sub-continental survey he would have done that which 
most men would consider notable and worthy of honour, yet it was 
carried out in a kind of "in-between" work, as a recreation from the 
more exacting toil of space-gauging and star-charting. 
When the part of Africa lying between Tanganyika and Upper Egypt 
is surveyed, we shall then know, with some approach to absolute accuracy, 
the length of an arc of meridian stretching from the Cape to Spitzbergen, 
that is through 110 degrees of latitude. 
The problem of stellar parallax very early interested Gill. Even in his 
Aberdeen days he hoped to obtain some results in this direction with his 
12-inch reflector, but of course the instrument could not respond to such 
a demand upon its accuracy. 
The success of his Mauritius and Ascension expeditions assured him 
that wuth a heliometer of larger size and superior construction it would 
be possible to determine differential parallax values within a margin of 
0-02 seconds of arc. 
In 1884 he ordered from Eepsold, of Hamburg, a new instrument of 
7 inches aperture. This heliometer was designed for a redetermina- 
tion of the sun's distance, and incidentally for determinations of stellar 
parallax. The instrument was completed and erected in 1887, and after 
the usual trials with it — testing its action, evaluating its errors — it was 
brought into regular use in 1888. 
In his researches on stellar parallax Gill was assisted first by Dr. 
Elkin and later on by Dr. de Sitter. Between them they determined the 
parallax of 22 stars with an accuracy never before attained. Gill took 
upon his own shoulders the major part of the observations, and his classic 
memoir on the parallax of Alpha Centauri and Sirius must ever remain an 
example of resolution, skill, and lucidity to all succeeding investigators in 
this difficult and exacting branch of astronomical research. 
His determinations, united to those of certain other astronomers in the 
Northern Hemisphere, afforded a theoretical value for the mean distance of 
first magnitude stars, a constant which enters very largely into the present- 
day investigations with regard to the structure and great cosmic move- 
ments of the stellar universe. If we assume that the stars are uniformly 
distributed through space, and that then- brightness and size also follow 
this general law of distribution, then there exists a very simple relation 
between the number of the stars, their brightness, and their distance. 
It is difficult to say what astronomer first cast doubts on the view of a 
uniform law of distribution ; but this much we do know — that from the 
moment Gill found that such stars as Canopus and Beta Centauri had 
