166 Britain's Heritage of Science 



in any position. In 1847 this instrument was at work, and 

 other important additions to the equipment were made as 

 occasion arose. Airy also originated the automatic system 

 by which the Greenwich time signals are transmitted each 

 day throughout the country. Among his theoretical investi- 

 gations in pure astronomy, one of the most important resulted 

 in the discovery of a new inequality in the motions of Venus 

 and the earth due to their mutual attraction, and this led to 

 an improvement in the solar tables. 



Sir John Herschel (1792-1871) was the only son of the 

 great astronomer whose work was considered in a previous 

 chapter. After graduating as senior wrangler in 1813, he 

 joined a number of friends in their efforts to reform the 

 teaching of mathematics at Cambridge. The astronomical 

 problems which had occupied the later years of Sir William's 

 life then attracted the son, who, after his father's death, 

 completed the work on double stars, and published an 

 important memoir on their orbits. 



In 1833 he embarked for the Cape, in order to extend to 

 the southern hemisphere the general survey of the heavens 

 which his father had carried out in the northern sky. It 

 was to a great extent a spirit of loyalty to his father which 

 kept him to the subject of astronomy, for his own bent of 

 mind drew him more towards physics and chemistry. He 

 discovered the solvent power of hyposulphite of soda on 

 otherwise insoluble salts of silver, a property which later 

 proved so useful in photography. As a writer he was clear 

 and effective. His article on " Light " in the Encyclopaedia 

 Metropolitana forms an excellent record of what was known 

 at the time, and his " Outlines of Astronomy " may still 

 serve as a useful book of reference. 



The work of Adams has already been described in a 

 previous chapter (p. 125). 



David Gill (1843-1914), after a period of study at the 

 University of Aberdeen, entered his father's business, 

 which consisted in the making of clocks. But his interest 

 in science, stimulated by the influence of Clerk Maxwell, 

 who for a time held a Professorship at Marischall College, 

 soon asserted itself, and he established a physical and chemical 

 laboratory in his father's house. Turning his attention to 



