THE ASTRONOMICAL VIEW OF NATURE. 329 



the study of the " mechanics of the heavens " by offering 

 large prizes for scientific and practical means of deter- 

 mining the longitude at sea. The lunar theory, which 

 has occupied the attention of the greatest mathematicians 

 since Newton of Euler, Clairaut, and Tobias Mayer in 

 the last century ; of Burckhardt, Plana, and Hansen, of 

 Delaunay and Adams, in the present century was an 

 outcome of this. It still engages the attention of scien- 

 tific minds, involving as it does all the most delicate 

 astronomical calculations, whilst for practical nautical 

 purposes the moon has ceased to be the great timekeeper, 

 and has since 1763 been replaced by the wonderful 

 chronometers of Harrison and his successors. A similar 

 stimulus both to abstract scientific research and to the 

 perfection of the practical instruments of measurement 

 was given in this century by the development of sub- 

 marine telegraphy : in this case both sides of the problem, 

 the scientific and the practical, were attacked, and carried 

 to a high degree of perfection by one and the same mind 1 



1 William Thomson's (Lord Kel- the signals and the gradual increase 



vin's) investigations and inventions, of the strength of the electric cur- 



which made submarine telegraphy rent at the receiving end of long 



at long distances commercially prac- [ submarine cables (" On the Theory 



ticable, refer mainly to the over- of the Electric Telegraph " and 



coming of the "embarrassment" other papers, reprinted in the 2nd 



occasioned by the property (dis- , vol. of 'Math, and Phys. Papers,' 



covered by Werner Siemens, 1849, 1884). The importance of con- 



and investigated by Faraday, 1854) ] structing delicate instruments for 



which submerged cables possess registering feeble signals, and of 



of "retaining a quantity of elec- ' a method for reducing the time 



tricity in charge along the whole of single signals, became evident 



surface." In 1854 Thomson made ; through these theoretical investi- 



a full theoretical examination of i gations. The mirror galvanometer 



this phenomenon, showed how it was first used in 1858 on the first 



depended on the length, the elec- Atlantic cable, and afterwards on 



trie resistance, and the electro- the successful cables of 1865 and 



static capacity of the line, and gave 1866. It was followed by the 



a mathematical formula, with prac- spark - recorder, which led to the 



tical examples of the retardation of syphon -recorder (1867-70), which 



