CHAPTER V. 



THE INNER PLANETS. 



MUCH progress has been made during the last 

 hundred years in our knowledge of the planets. 

 In fact, the study of Mercury only dates from 

 the commencement of the nineteenth century. 

 Our knowledge of the vicinity of the Sun is 

 very limited, and Mercury is difficult of observa- 

 tion. So limited, in fact, is our knowledge of 

 the Sun's surroundings, that it is not yet known 

 for certain 'whether there is a planet, or planets, 

 between Mercury and the Sun. Perturbations 

 in the motion of the perihelion of Mercury's 

 orbit led Le Verrier in 1859 to the belief that 

 a planet of about the size of Mercury, or else 

 a zone of asteroids, existed between Mercury and 

 the Sun. It was, however, obvious that such a 

 planet could only be seen when in transit across 

 the Sun's disc, or during a total eclipse. Mean- 

 while a French doctor, Lescarbault, informed 

 Le Verrier that he had seen a round object in 



