58 The Ottawa Naturalist. [June-July 



brief statement of the phenomena in agreement and disagree- 

 ment with the theory may be of interest. In agreement: 



1 . The planets all revolve nearly in the same plane and in 

 the same direction. 



2. Their orbits are all nearly circular. 



3. The sun and the planets, so far as known, rotate in the 

 direction in which the planets revolve. 



4. The planes of the equators of the planets and of the 

 orbits of their satellites are nearly coincident with the planes 

 of their orbits (Uranus and Neptune excepted). 



5. The satellites revolve in the direction that the primaries 

 rotate (9th of Saturn and 8th of Jupiter exceptions). 



6. According to the contraction theory of the sun's heat, this 

 body was once vastly larger than at present. 



Some facts inconsistent with the Nebular Theory. : 



1. The orbits of the asteroids are contradictorv to the 

 theory. 



2. The rapid revolution of the inner satellite of Mars and 

 of the particles of the inner ring of Saturn can not be satisfactor- 

 ily explained. 



3. The presence of light elements in the earth is not to be 

 expected. 



4. A series of rings could not have been left off. 



5. A ring could not have condensed into a planet. 



6. The retrograde revolutions of the 9th satellite of Saturn 

 and the 8th of Jupiter contradict the theory. 



Various modifications of Laplace's theory to meet these 

 objections have been brought forward by Roche, Faye, Ligoudes, 

 Ball and others, the chief of which dispenses with the trouble- 

 some process of ring formation and condensation and starts 

 the planets by condensations around accidental nuclei in the 

 parent nebula. The most exhaustive criticism of the nebular 

 theory is that by Moult on and Chamberlin, published in 1900, 

 who, by combining observed facts with dynamical principles 

 show that in its present form it fails to account for many of the 

 phenomena of the Solar System. 



An alternative hypothesis has been developed by these 

 writers called the Planetesimal or Spiral Nebula Hypothesis. It 

 probably owes its origin to the fact that the researches of Keeler 

 with the Crossley Reflector at the Lick Observatory showed 

 that the predominant, form of nebula was the spiral, and that no 

 known nebula has a form agreeing with Laplace's Ring Hypo- 

 thesis. 



The authors of this theory assume that our system was 

 originally a small spiral nebula and explain the formation of the 

 spiral nebulae by the collision theory al^adv dealt with, or 



