ASTKONOMY. 33 



was followed until February 11, 18*78. The disappearance 

 of the ring occurred about February 6. The angle of posi- 

 tion of the major axis of the ring was observed on thirty-six 

 nights by Professor Hall, and on twenty-two nights by Pro- 

 fessor Holden. Although at the time of the disappearance 

 of the ring the planet was too near the sun for good obser- 

 vations, yet the whole of these observations indicate that 

 Bessel's elements of the ring are very nearly correct." 



Professor Hall, of Washington, lias an investigation of the 

 outline of the shadow of a planet projected on any plane first, 

 for the case where the luminous and opaque bodies are both 

 spherical; and, second, where the opaque body is supposed to 

 be an ellipsoid of revolution. The conclusion is that, even 

 in the case of Saturn, which has the most eccentric figure of 

 any of the planets, the outline of the geometric shadow on 

 the plane of the ring is sensibly a right line. The apparent 

 convexity of the bounding line of this shadow towards the 

 centre of Saturn has then to be explained from conditions 

 other than geometrical. 



M. Souillart, known by his researches on the theory of 

 Jupiter'' 8 satellites, has a paper in the Astronomische Nacli- 

 richten on the shape of the shadow of a planet, and comes to 

 essentially the same conclusions as previously given by Pro- 

 fessor Hall in the same journal. 



M. Tisserand, of Toulouse, who has lately occupied himself 

 with the system of Saturn,h&s an important note in Compter 

 Rendus on the nature of the ring. Laplace proved in 1787 

 that even if observation did not show that the ring of Saturn 

 was composed of two or more concentric rings, the theory of 

 gravitation would require this. Tisserand, as the result of 

 the re-examination of the problem, comes to the conclusion 

 that a continuous ring of the dimensions of the real rinse c^ 11 ' 

 not exist in equilibrium. Hence it is divided. In fact, the 

 ring of Saturn has been seen (by Bond, De la Rue, Dawes, 

 etc.) divided into numerous fine concentric rings, just as this 

 condition requires. 



Uranus and Neptune. The satellites of these planets are 

 followed at the Naval Observatory, Washington. 



B2 



