, LAPLACE. 163 



With respect to the zodiacal light, that rock against which so mauy 

 reveries have beeu wrecked, it cousists of the most volatile parts of the 

 primitive nebula. These molecules, not having united with the equa- 

 torial zones, successively abandoned in the plane of the solar equator, 

 continue to revolve at their original distances, and with their original 

 velocities. The circumstance of this extremely rare substance being 

 included wholly within the earth's orbit, and even within that of Venus, 

 seemed irreconcilable with the principles of mechanics; but this diffi- 

 culty occurred only Avhen the zodiacal substance being conceived to be 

 in a state of direct and intimate dependence on the solar i^hotosphere, 

 l)roperly iso called, an angular movement of rotation was imiiressed on 

 it equal to that of the photosphere, a movement in virtue of which it 

 effected an entire revolution in twenty-five days and a half. Laplace 

 presented his conjectures on the formation of the solar system with the 

 diffidence inspired by a result which was not founded upon calculation 

 and observation.* 



Perhaps it is to be regretted that they did not receive a more complete 

 development, especially in so far as it concerns the division of the mat- 

 ter into distinct rings; perhaps it would have been desirable if the illus- 

 trious author had expressed himself more fully respecting the primitive 

 physical condition, the molecular condition of the nebula at the expense 

 of which the sun, planets, and satellites of our system were formed. It 

 is perhaps especially to be regretted that Laplace should have only 

 briefly alluded to what he considered the obvious possibility of move- 

 ments of revolution having their origin in the action of simple attractive 

 forces, and to other questions of a similar nature. 



Notwithstanding these defects, the ideas of the author of the Meca- 

 nique Celeste are still the only speculations of the kind which, by their 

 magnitude, their coherence, and their mathematical character, may be 

 justly considered as forming a physical cosmogony; those alone which 

 in the present day derive a powerful support from the results of the re- 

 cent researches of astronomers on the nebulte of every form and magni- 

 tude which are scattered throughout the celestial vault. 



In this analysis, we have deemed it right to concentrate all our atten- 

 tion upon the Mecanique Celeste. The Sysfeme du Monde and the 

 Theorie Analytique des Prohahilitcs would also require detailed notices. 



The Exposition du Isysteme du Monde is the Mecanique Celeste divested 

 of the great apparatus of analytical formula which ought to be atten- 

 tively perused by every astronomer who, to use an expression of Plato, 

 is desirous of knowing the numbers which govern the physical universe. 

 It is in the Exposition du Systeme du Monde that persons unacquainted 

 with mathematical studies will obtain an exact and competent knowl- 

 edge of the methods to which physical astronomy is indebted for its as- 

 tonishing progress. This work, written with a noble simplicity of style, 



* Laplace has explaiued this theory in his Exposition du Systeme du Monde, (liv. 4, uote 

 7.)— Translator, 



