168 Britain's Heritage of Science 



instrument of his, the potentialities of which for accurate 

 measurements he was the first to recognize, was the helio- 

 meter, the essential par of which consists of an object-glass 

 divided into two halves, which could be made to slide along 

 the dividing line. If the image of a star formed by one half 

 be brought into coincidence with the image of a neigh- 

 bouring star formed by the other half, the angular distance 

 between the stars is indicated by a suitable measuring 

 arrangement. With a telescope of this construction Gill 

 instituted a series of observations for the determination of 

 stellar parallaxes, which raised the subject up to a higher 

 plane. Another important research carried out by Gill with 

 the assistance of others was the determination of the mass of 

 Jupiter by observations of his satellites. 



Gill was not only an eminent investigator; large ideas 

 originated in his mind, and were pushed forward with 

 unlimited energy. He originated the great international 

 enterprise for cataloguing and charting the whole sky by 

 photography. He also successfully advocated an accurate 

 trigonometrical survey of the whole of South Africa, and 

 formed a scheme for the measurement of an arc of meridian 

 which should run along the thirtieth meridian east of Green- 

 wich through the whole length of Africa to the mouth of the 

 Nile, and connect by triangulation through the Levant with 

 the Roumanian and Russian arcs. He secured the assistance 

 of Mr. Cecil Rhodes, and the work, though frequently inter- 

 rupted, partly through the political troubles in Africa and 

 partly through want of money, was proceeding slowly when 

 stopped by the outbreak of the present war. 



Gill's scientific activity was continued after his return to 

 England, and during the last years of his life he endeavoured 

 to stimulate the manufacture of optical glass in this country. 

 His efforts deserved a better response than they received 

 and though they were primarily directed towards securing 

 the large blocks required for telescopes, the whole question 

 of glass manufacture, which has since become of such pressing 

 importance, was involved. By his death British science lost 

 an intensive driving force. 



While professional astronomers carried on their excellent 

 researches the great improvements in the construction of 



