Sir David Gill. 
201 
a value only O'OTs. removed from a more refined and less circuitous deter- 
mination made in 1908, thirty years later. 
During his journey home Gill, at the request of the Egyptian Govern- 
ment, spent some months in Egypt measuring a hase line there. This 
was his first introduction to geodetic work, but one reading the account 
of the investigation would utterly fail to discover a trace of immaturity 
in the mode of approach adopted, or a shadow of uncertainty in the 
conclusions arrived at. 
Gill all through his life was drawn tow^ards work of this character, 
for, though he held Herschel before him as a model to follow, his mind 
always gravitated towards the more refined branches of observational 
astronomy. He was in intellectual outlook rather the disciple of Gauss 
and Bessel and Struve than of Herschel and Bond. He had in any 
research depending largely upon the uttermost accuracy of instrumental 
adjustments an inspired insight into the best methods to be adopted 
in order to arrive at a satisfactory result ; and when once his mind 
was quite clear as to the way he should go, he went. 
The success of the Mauritius expedition made him eager to attempt 
again the problem of the sun's distance. 
In 1877 there occurred a very favourable opposition of the planet 
Mars. On September 5th of that year Mars came within 34,000,000 
miles of the earth, and thus there was afforded an exceptional opportunity 
of securing an accurate value of the sun's distance. There was consider- 
able difficulty in obtaining the necessary funds for the expedition, and 
when this was arranged its prospects were nearly ruined by an accident 
to the heliometer. 
The instrument was set up in the rooms of the Eoyal Astronomical 
Society, in order that it might be thoroughly tested. Through the 
insufficiency of one of the holding screws, one day the whole instru- 
ment tilted over, coming down on its eye-end, which was driven right 
through the floor by the force of tbe fall. Tradition still points to the 
place where the accident happened. Fortunately the damage was far 
less than what the horror-struck astronomer expected. Gill relates that 
for a few minutes he w^as afraid to touch the instrument, afraid to look at 
its bent and broken adjustments. 
In ten days compatent instrument-makers repaired the mountings, 
and soon Gill w^as on his way to Ascension to carry out an investigation 
w^hich will ever remain a classic in astronomical endeavour. There w^ill 
no doubt be more accurate determinations of the sun's distance — Gill's 
own later researches gave more reliable results : there will certainly 
be more elaborate and more highly organized investigations in the field 
of stellar parallaxes; but as the single achievement of one man, as a 
complete and admirably reasoned investigation, in which every possible 
