346 LAPLACE. 



observation, he abandoned to Huyghens the honour of 

 being regarded as the author of the true theory of the 

 phenomena presented by the wonderful planet. 



Every person knows, in the present day, that Saturn 

 consists of a globe about 900 times greater than the 

 earth, and a ring. This ring does not touch the ball of 

 the planet, being everywhere removed from it at a 

 distance of 20.000 (English) miles. Observation indi- 

 cates the breadth of the ring to be 54,000 miles. The 

 thickness certainly does not exceed 250 miles. With 

 the exception of a black streak which divides the ring 

 throughout its whole contour into two parts of unequal 

 breadth and of different brightness, this strange colossal 

 bridge \vithout piles had never offered to the most expe- 

 rienced or skilful observers either spot or protuberance 

 adapted for deciding whether it was immovable or 

 endued with a movement of rotation. 



Laplace considered it to be very improbable, if the 

 ring was immovable, that its constituent parts should be' 

 capable of resisting by their mere cohesion the continual 

 attraction of the planet. A movement of rotation oc- 

 curred to his mind as constituting the principle of sta- 

 bility, and he hence deduced the necessary velocity. 

 The velocity thus found was exactly equal to that which 

 Herschel subsequently deduced from a course of ex- 

 tremely delicate observations. 



The two parts of the ring being placed at different 

 distances from the planet, could not fail to experience 

 from the action of the sun, different movements of rota- 

 tion. It would hence seem that the planes of both rings 

 ought to be generally inclined towards each other, 

 whereas they appear from observation always to coin- 

 cide. It was necessary then that some physical cause 



