THE SUN. 61 



the uncondensed vapours and gases which form 

 the atmosphere in which the clouds of the 

 photosphere are suspended." He says that the 

 contraction theory of Helmholtz, explained in 

 another chapter, advanced to explain the main- 

 tenance of the Sun's heat, is true so far as it 

 goes ; but that it is all the truth is now made 

 doubtful by the discovery of radium, which 

 " suggests that other powerful sources of energy 

 may co-operate with the mechanical in main- 

 taining the Sun's heat." 



The important question of the distance of the 

 Sun was thoroughly investigated in 1824 by 

 Johann Franz Encke (1791-1865), then of See- 

 berg, near Gotha, who, from a discussion of the 

 transits of Venus in 1761 and 1769, found a 

 parallax of 8"*371, corresponding to a mean 

 distance of 95,000,000 miles. This value was 

 accepted for thirty years, until Peter Andreas 

 Hansen (1795-1874), in 1854, and Urban Jean 

 Joseph Le Verrier (1811-1877), in 1858, found 

 from mathematical investigations that the dis- 

 tance indicated was too great. Preparations 

 were accordingly made for the observation of 

 the transits of Venus, which took place respect- 

 ively on December 8, 1874, and December 6, 

 1882. On the first occasion many expeditions 

 were sent to view the transit, consisting of 



