PRESIDENT S ADDRESS. 9 



Jtirst essay at the problem was made at Mauritius in 1874. The 

 method he adopted was that of finding the parallax of Juno, and, 

 consequently, of the sun, by taking diurnal observations of the 

 planet. The experience gained in this expedition led him to a 

 second determination. The series of observations of Alars, 

 which was the planet used as " intermediary " between the earth 

 and the sun, resulted in a value of the sun's parallax equal to 

 93,080,000 miles. 



Unsatified even with this result— what true scientist is ever 

 satisfied with his results? — Gill, in 1888, organized a combined 

 attack on the problem. Five Northern observatories and one 

 Southern entered upon a heliometric campaign against the 

 minor planets Iris, Victoria and Sappho. The observations 

 secured were reduced at the Cape, which throughout formed the 

 headquarters of the investigation. The work done, its magni- 

 tude, its importance, its finality, have been already referred to. It 

 was not till 1897, ten years after embarking on the campaign, 

 that Gill was able to announce a distance of 92,875,000 miles as 

 the result of his labours. 



In more recent years two other notable determinations of 

 this fundamental constant of astronomy have been secured, both 

 by novel methods. 



I.ast year, Hinks, of Oxford, from photographic measures 

 of Eros, taken also at a number of observatories, found a dis- 

 tance of 92,826,000 miles, and four years ago, Halm, at the Cape, 

 by spectroscopic measures of stars in the line of sight, arrived at 

 a value of the solar parallax equal to 92,896,000 miles. Com- 

 bining these three determinations, obtained by radically diverse 

 methods, we obtain a value ot the solar parallax equal to 

 92,866,000 miles. 



In any determination of the sun's distance three other con- 

 stants come within the radius of the investigation: the moon's 

 distance and mass, for it is the distance of the centre of gravity 

 of the earth-moon system that has to be finally determined ; the 

 constant of aberration, that is, the relation subsisting between the 

 solar parallax and the velocity of light; and the oblateness of the 

 earth's form. 



Lacaille, one hundred and seventy years ago, determined, 

 from his meagre home in Strand Street, the moon's distance, the 

 aberration constant, and also set out on this southern land the 

 first geodetic base line. The present Astronomer Royal at the 

 Cape, faithful to these old traditions, is covering unexplored 

 Northern Rhodesia with a network of triangulations, in hope, one 

 day, of carrying the line that Lacaille began to the confining 

 northern sea, and thus linking it on to the great web of European 

 stations. It seems strange to connect the measuring of an insig- 

 nificant base line out on the Cape Flats with spacious sidereal 

 soundings, but there is an unbroken sequence in endeavour and 



