306 RAMBLES OF A NATURALIST. 



his Optics and his Principia ; and on the latter of these 

 rests his principal title to fame. In this work he has 

 developed, in a manner which all astronomers declare to 

 be truly admirable, the doctrine of universal attraction, 

 a doctrine which has furnished a key to a host of pheno- 

 mena, and on which the entire system of modern astro- 

 nomy is based. Without going too much into details 

 which would carry us beyond the plan of this work, we 

 will briefly recapitulate the principles established by 

 Newton. 



1st. All the particles of matter diffused through the 

 universe mutually attract one another directly as their 

 mass, and inversely as the square of the distance. 



2nd. This force is independent of time, it acts through 

 all substances whatever may be their nature, and their 

 state of rest or motion. 



3rd. When two spherical bodies attract one another, 

 the attraction is exerted precisely as if the entire mass 

 were concentrated in the centre of each sphere, and 

 consequently as if each of them formed only one sole 

 particle. 



4th. Two spherical bodies, obedient to the action of 

 attraction, describe each round the other, considered as 

 fixed, concave curves belonging to conic sections: they 

 likewise describe similar curves round their common 

 centre of gravity. These curves will be ellipses, para- 

 bolas, or hyperbolas according to the conditions which 

 are present. 



5th. In each case, the angular velocity with which 

 the line joining the centres moves will be in an inverse 

 ratio to the square of their mutual distance, and the areas 

 described by this line will be equal in equal times. 



For a long time experience has proved that these 

 laws govern our solar system, and recent discoveries 

 on the motions of double stars have proved that they 



