636 
Ascension and the Cape. The details of these 
and other instruments were worked out, domes 
planned and built, and the telescopes mounted 
and brought into working order. 
Lord Lindsay had arranged to observe the 
transit of Venus of 1874 in the island of 
Mauritius, and the task of determining the longi- 
tude of his station was assigned to Gill. Aden 
was connected with Greenwich by telegraph, but 
for the connection of Mauritius with Aden it was 
necessary to carry chronometers. No fewer than 
forty chronometers were taken and carried by 
Gill single-handed to their destination and back, 
a task of great anxiety and difficulty, especially 
at embarkation or landing at places like Suez, 
Alexandria, Aden, and Mauritius, where only 
coloured labour was available. A series of excel- 
lent determinations of longitude were obtained, 
and on the return journey the measurement of the 
base-line for the Egyptian Survey was made, the 
site selected being nearly in front of the Sphinx. 
The expedition to Mauritius was memorable in 
another way. Though hampered by cloudy weather, 
Gill and Lindsay determined the solar parallax 
from a short series of heliometer observations of 
the minor planet Juno, and demonstrated the high 
value of this method. This was followed up by 
an expedition to the island of Ascension to utilise 
the opposition of Mars in 1877 for the same 
purpose. Gill having given up his connection 
with Dun Echt, Lord Lindsay granted him the 
loan of the 4-in. heliometer; the cordial support 
of the Royal Astronomical Society assured the 
necessary financial assistance, afterwards defrayed 
by the Government Grant Fund of the Royal 
Society. A delightful account of this expedition 
is given in “Six Months in Ascension, by 
Mrs. Gill—an unscientific account of a scientific 
expedition.” An excellent determination of the 
solar parallax was obtained, and it was shown 
that for still higher accuracy it would be necessary 
to utilise the opposition of a minor planet owing 
to the observational uncertainty in setting on 
the limb of a planet with a perceptible disc. 
On February 10, 1879, Gill was appointed 
H.M. Astronomer at the Cape. After a few 
months spent in visiting the principal observa- 
tories in Europe, he proceeded to the Cape, 
arriving there on May 26. The Cape Observatory 
had, under Gill’s predecessors, Fallowes, Hender- 
son, Maclear, and Stone, accomplished valuable 
work in the determination of the positions of the 
stars of the southern hemisphere. This import- 
ant work, which falls naturally to large national 
observatories, was continued by him. He reduced 
and published the observations made by Maclear 
during the years 1849-52 and 1861-70, thus 
clearing off all arrears in the publication of the 
Cape observations. During his directorate he 
published catalogues of the fundamental stars 
observable at the Cape, of zodiacal stars the 
positions of which are required in heliometer and 
other observations of the moon and planets, and 
of 8560 stars to serve as reference points for the 
photographs in- the section of the international 
NO. 2310, VOL. 92| 
NATURE 
‘observations 
(cpetes 5, 1914 
photographic chart and catalogue undertaken by 
the Cape. He improved and carefully studied 
the details, such as pivot and circle errors, of 
the transit circle which had been erected in 1856. 
But he strongly held to the view that a reversible 
instrument was necessary for fundamental work 
of the highest accuracy, and when the purchase 
of such an instrument had been sanctioned by 
the Admiralty, threw his whole energy and 
mechanical and engineering skill into making 
the instrument the best of its kind. A brief 
account of its most striking features is given in 
Nature for January 15, p. 556. It was only com- 
pleted at the time of Gill’s retirement from the 
Cape in 1906, but the results obtained by his suc- 
cessor, Mr. Hough, show that it has admirably 
fulfilled the object of high accuracy and freedon) 
from systematic error. 
Knowing what effective use he would be able 
to make of the 4-in. heliometer, Gill acquired it 
from Lord Crawford, and took it with him to 
the Cape. He employed it first in the determina- 
tion of the parallaxes of nine southern stars which 
were remarkable for their great brilliancy or the 
size of their proper motions. In this task he was 
joined by Mr. Elkin, a young astronomer whose 
acquaintance he made at Strassburg in 1879- 
The valuable results obtained by the two observers 
were published in 1884. After the execution of 
the work, Gill pointed out to the Lords Com- 
missioners of the Admiralty that a larger instru- 
ment was necessary for the further prosecution 
of research in stellar and solar parallax, and re- 
ceived their sanction for the purchase of a 7-in. 
heliometer. With the new instrument the paral- 
laxes of twenty-two southern stars were deter- 
mined with the highest accuracy. The work 
entailed extremely delicate and careful 
shortly after sunset and before 
sunrise extending over many months, and, in 
addition, laborious researches on the values and 
errors. of screws and scale-divisions. This re- 
search, in which Gill’s personal observations 
were supplemented by those of Finlay and de 
Sitter, has been recognised as the high-water mark 
of astronomical observation, and will probably 
never be surpassed by visual observations. 
For the determination of the solar parallax 
Gill found that the minor planet Iris would be 
very favourably situated in 1887, and Victoria 
and Sappho in 1888. He determined to make 
observations himself, and secured promises of 
cooperation from other astronomers who pos- 
sessed heliometers, and also of meridian observa- 
tions to secure an accurate framework for the 
positions of the necessary reference stars. A very 
extensive programme was carried out, and the 
observations are discussed in two large volumes 
of the Cape Annals. The value of the solar paral- 
lax was found to be 8/°804, with a probable error 
of only +0”-0046. This result has been recently 
confirmed by the photographic observations of 
the planet Eros, and still more recently from the 
spectroscopic observations of the differences of 
the velocities of stars in the line of sight when the 
