PERTURBATION OF URANUS. 



when at D is in heliocentric conjunction with the supposed planet, 

 the latter being then at d. 



The directions of the orbital motions of the two planets are 

 indicated by the arrows beside their paths ; and the directions of 



Pig. 2. 



the disturbing forces * exercised by the supposed planet on Uranus 

 are indicated by the arrows beside the lines joining that planet 

 with Uranus. 



Now, it will be quite evident, that the attraction exerted by 

 the supposed planet at a on Uranus at A tends to accelerate 

 the latter. In like manner, the forces exerted by the supposed 

 planet at b and c upon Uranus at B and c tend to accelerate 

 it. But as Uranus approaches to D, the direction of the dis- 

 turbing force, being less and less inclined to that of the orbital 

 motion, has a less and less accelerating influence, and on 

 arriving at D, the disturbing force being in the direction D d 

 at right angles to the orbital motion, all accelerating influence 

 ceases. 



After passing D the disturbing force is inclined against the 

 motion, and instead of accelerating retards it; and as Uranus 

 takes successively the positions E, P, &c., it is more and more 

 inclined, and its retarding influence more and more increased, as 

 will be evident if the directions of the retarding force and the 

 orbital motion, as indicated by the arrows, be observed. 



* To simplify the explanation, the effect of the attraction of Uranus on 

 the sun is omitted in this illustration. In the chapter on Perturbations in 

 my "Handbook on Astronomy," the method of determining the exact 

 direction of the disturbing force is explained. 



LARDNER'S MUSEUM OF SCIENCE. N 177 



No. 77. 



