aro 
346 Ais 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 aring. 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- 
eates 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 without 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 
