ASTRONOMY AND METEOROLOGY. 337 



sued this subject for the last ten years, it having been first suggested 

 to him by the nebular hypothesis, which he thought could be estab- 

 lished by some law of rotation. 



Mr. Walker then entered into a lengthened examination of the data 

 on which the law rests, and seemed to come to the conclusion, that, as 

 Jar as we know at present, everything is in favor of the truth of the law, 

 except that it requires the assumption of another planet between Jupi- 

 ter and Mars. 



Mr. Walker closed j^is examination by saying, " We may therefore 

 conclude, that, whether Kirkwood's analogy is or is not the expression of 

 a physical law, it is at least that of a physical fact in the mechanism of the 

 universe. The quantity on which the analogy is based has such imme- 

 diate dependence upon the nebular hypothesis, that it lends strength to 

 the latter, and gives new plausibility to the presumption that this, also, 

 is a fact in the past history of the solar system. 



" Such, then, is the present state of the question. Thirty-six ele- 

 ments of nine planets (four being hypothetical) appear to harmonize 

 with Kirkwood's analogy in all the four fundamental equations of con- 

 dition for each planet. To suppose that so many independent variable 

 quantities should harmonize together by accident, is a more strained 

 construction of the premises than the frank admission that they follow 

 a law of nature. 



" If, in the course of time, the hypotheses of Laplace and Kirkwood 

 shall be found to be laws of nature, they will throw new light on the 

 internal organization of the planets, in their present and in any more 

 primitive state through which they may have passed. For instance, we 

 may compute the distance from the centre at which any planet must 

 have received its projectile force in order to produce at the same time 

 its double movement of translation and rotation. 



" If the planet in a more primitive state existed in the form of a ring 

 revolving round the sun, having its present orbit for that of the centre 

 of gravity of the ring, the momentum of rotation must, by virtue of 

 the principle of conservation of movement, have existed in some form 

 in the ring. It is easy to perceive that this momentum is precisely the 

 amount which must be distributed among the particles of the ring, in 

 order to preserve to all the condition of dynamical equilibrium, while 

 those of each generating surface of the ring were wheeling round with 

 the same angular velocity. 



" If the planets have really passed from the shape of a revolving ring 

 to their present state, the prevalence of Kirkwood's analogy shows a 

 nice adaptation of parts in every stage of the transition. 



"If the primitive quantity of caloric (free and latent) had undergone 

 a very great change beyond that now indicated in the cooling of their 

 crusts; if the primitive quantity of movement of rotation had been dif- 

 ferent from its actual value for any planet; if the law of elasticity of 

 particles for a given temperature and distance from each other varied 

 from one planet to another in the primitive or present states ; in either 

 of these cases, the analogy of Kirkwood might have failed. As it is, 

 no such failure is noticed ; we are authorized, therefore, to conclude, 

 that the primitive quantity of caloric, the law of elasticity, the 



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