PHILOSOPHICAL SOCIETY OF WASHINGTON. 191 



The periodic time of a planet is given by ttie formula — ^ 



Periodic time' = r^rean disiance' x comtant 



muss 0/ sun -\- mass q/ planet. 



By aerolithic increase of the mass of the sun, and consequent 

 augmentation of centripetal force, the planet is drawn inward, 

 and its mean distance diminished. The periodic time is there- 

 fore diminished, both by decreasing the numerator and increasing 

 the denominator of the above fraction. 



Aerolithic increase of the mass of a planet has a similar effect; 

 and the mean distance is generally still further diminished by 

 resistance to the motion of the planet. It has been ascertained 

 by observation that the earth meets more aerolites than overtake 

 it ; and it may reasonably be assumed that the absolute motions 

 of all the aerolites encountered by a planet nearly neutralize each 

 other, and that the resultant effect on the motion of the planet ia 

 nearly the same as if. the entire addition to its mass had previ- 

 ously been absolutely at rest. In a similar manner these addi- 

 tions to tlie mass of the sun diminish the velocity of its revolu- 

 tion around its own axis. 



Similar reasoning is evidently applicable to the case of planet 

 and satellite. In all these ways the relative velocity of a satellite 

 in its revolution around its primary is increased as compared with 

 the velocity of the axial revolution of the latter. 



From Dr. von Asten's computations, it appears that Encke's 

 comet encounters an irregular resistance, sometimes returning to 

 perihelion as if it had encountered no resistance whatever. The 

 uniform resistance of the luminiferous ether or of a cosraical atmo- 

 sphere seems therefore to be utterly inappreciable ; and it may 

 be inferred that if there is any such resistance, it is of small im- 

 portance as compared v^^ith aerolithic disturbance. 



If the inner moon of Mars was formed from that planet, in 

 accordance with Laplace's nebular hypothesis, and has since been 

 brought to its present position solely or principally by aerolithic 

 action, it is evident that a very large addition has been made in 

 that manner to its original mass. It is suggested that the eccen- 

 tricity of the planetary orbits, and the want of coincidence between 

 the planes of the orbits and equators of the planets and the plane 

 of the solar equator, is perhaps better explained by the action of 

 aerolites than by differences of temperature and density, as sup- 

 posed by Laplace. These considerations also indicate immense 

 additions to the original planetary masses. 



It is certain that unmodified meteoric stones form but a very 

 small part of the accessible matter of the earth ; and it may thence 

 be inferred that by far the greater part of this aerolithic acquisi- 

 tion was made during an immensely long cosmological period 

 before the earth solidified into a record-keeping condition. But 

 it is hardly safe to assume that the chemical constitution of. the 

 meteoric stones resembles that of shooting stars in general ; and 

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