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 I^ewton 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 and has since 1763 been replaced by the wonderful 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 William Thomson's (Lord Kel- 

 vin's) investigations and inventions, 

 which made submarine telegraphy 

 at long distances commercially prac- 

 ticable, refer mainly to the over- 

 coming of the " embarrassment " 

 occasioned by the property (dis- 

 covered by Werner Siemens, 1849, 

 and investigated by Faraday, 1854) 

 which submerged cables possess 

 of " retaining a quantity of elec- 

 tricity in charge along the whole 

 surface." In 18.54 Thomson made 

 a full theoretical examination of 

 this phenomenon, showed how it 

 depended on the length, the elec- 

 tric resistance, and the electro- 

 static capacity of the line, and gave 

 a mathematical formula, with prac- 

 tical examples of the retardation of 



the signals and the gradual increase 

 of the strength of the electric cur- 

 rent at the receiving end of long 

 submarine cables ("On the Theory 

 of the Electric Telegraph " and 

 other papers, reprinted in the 2nd 

 vol. of ' Math, and Phys. Papers,' 

 1884). The importance of con- 

 structing delicate instruments for 

 registering feeble signals, and of 

 a method for reducing the time 

 of single signals, became evident 

 through these theoretical investi- 

 gations. The mirror galvanometer 

 was first used in 1858 on the first 

 Atlantic cable, and afterwards on 

 the successful cables of 1865 and 

 1866. It was followed by the 

 spark -recorder, which led to the 

 syphon -recorder (1867-70), which 



