BiOKBETON. — Causes tending to alter Eccentricity of Ptajietary Orbits. 153 



On the Accretion of Particles. 



When the nebula has become stable, that is no longer expanding, much 

 of its elementary matter will be at a sufficiently low temperature to com- 

 bine ; and these compound molecules will tend to aggregate into groups, 

 and it may be shown that these masses may ultimately become so consider- 

 able as to form star clusters, associated by gravitation but not coalescing. I 

 propose discussing this difficult question in a paper on star clusters ; but in 

 this paper I shall simply state that these little bodies will in all probability 

 revolve in independent orbits at aU eccentricities around the centre of the 

 mass ; these and other masses will be occasionally picked up by the planet. 

 The following problem shows that component to and from the centre 

 tends to be destroyed, and only a circular orbit left. 



Prohleyn 4l. — Given two bodies revolving in eccentric orbits in the same 

 direction around an attracting centre, to find the effect upon the eccentri- 

 city of the new orbit in the event of their coalescing. — Let a represent the 

 two bodies colliding, the direction of one body is along the path ab, the other 

 along ac, the component along the diagonal ad is evidently the new direc- 

 tion of the coalesced body ; it has evidently less velocity than the mean of 

 the two, as the component towai'ds and away from the sun is more or less 

 destroyed. The position of the body is also nearer after than before, so 

 that the total effect will be to reduce mean distance, to lessen aphelion dis- 

 tance, and generally to increase perihelion distance, in other words to lessen 

 eccentricity. 



There can be but little doubt that this agency of accretion will be most 

 important in giving regularity to any system. Proctor has discussed the 

 influence of accretion of meteors, and it is certain he is right in giving it 

 very great value. It probably played a great part in the formation of 

 Jupiter. 



Uncertain Agencies. 



An agency whose effect it is difficult to estimate is that of the outward 

 motion of the general mass of the nebula. The planet may meet this on its 

 return towards the centre ; if so it will directly oppose its return, acting 

 exactly opposite to gravitation, that is, in the same manner as though the 

 central mass were of less mass ; the body will consequently not be attracted 

 back towards the centre so far as it otherwise would be. This action will 

 tend to lessen perihelion distance. This outrush wiU evidently be much 

 less as the planet leaves the sun on its second revolution, thus the body will 

 not be aided by it on its return, and consequently it may not reach its full 

 aphehon ; but it is extremely likely that the nebula will attain equilibrium 

 before it could affect the planet on its return. In the event of a case of 

 partial impact in which the two parts of the original bodies escaped into 



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