1884.] lUV) [Khkword. 



present distance in a nebular state, it must follow, if any form of the 

 nebular hypothesis is to be accepted, that its original distance was greater 

 than the present. Can we assign a probable cause for this ancient dis- 

 turbance ? 



Of the eight major planets. Mars has the most eccentric orbit, except 

 that of Mercury ; its perihelion distance being 13,000,000 miles less than 

 its mean distance. This difference, in fact, amounts to 20,000,000 miles 

 when the orbit of Mars has its greatest eccentricity. If, therefore, the 

 radius of the sun, or of the solar atmosphere, was somewhat greater than 

 the least distance of Mars at the commencement of the latter's separate 

 existence, the j)lanet in perihelion would pass through the outermost 

 equatorial zone of the solar nebula. This resisting medium would not 

 only accelerate the motion of Mars, but also, in a much greater degree, 

 that of his extremely small satellites. The solar volume, meanwhile con- 

 tracting more rapidly than the orbit of Mars, would finally leave the latter 

 moving in an eccentric path, without sensible resistance.* 



IV. T7ie Saturnian System. — For Mimas, the first satellite of Saturn, the 

 most probable values of the mass and density give the distance of the limit 

 from the satellite's surface less than the radius of Mimas. The rings of 

 Saturn, in all probability, could not exist as three satellites, the limits ot 

 equilibrium being interior to the surface. This is true at least in the case 

 of the innermost ring. Analysis seems to indicate that planets and 



COMETS HAVE NOT BEEN FORMED FROM RINGS, BUT RINGS FROM PLANETS 



AND COMETS. If, wilhout any loss of mass, the density of a planet were 

 diminished until the radius should exceed the limit of equilibrium, what 

 change would take place in the planetary form? Evidently a portion ot 

 the matter nearest the central body would be separated from the rest, and, 

 as the orbital velocity would be less than that corresponding to its dis- 

 tance, it would move in a new ellipse, the aphelion of which would be the 

 point of separation. 



V. Comets. — The effect of the sun's attraction in the dismemberment 

 of comets is well known to astronomers. The nuclei of the large comets 

 of 1680, 1843, 1880 and 1882 must have had great force of cohesion be- 

 tween their parts, in order to withstand the tendency to disintegration at 

 the times of perihelion passage. Had the nuclei been either liquid or 



* This view was first presented in the Observatory for January, 1878. DiflTerent 

 explanations of the sliort period of Phobos have been proposed by astronomers, 

 but none, perliaps, entirely free from difflcnlties. One distinguished writer has 

 suggested tliat7h.39m, the period of Phobos, was the rotation period of Mars at 

 tlie epocli of the satellite's origin, and that the lengthening of the period to 

 24h 37m has been due to retardation by solar tides. But it is well known that the 

 lime of rotation of a planet in tlie process of condensation varies as the square 

 of its radius. Tlie resulting period of Mars, therefore, on reaching its present 

 dimensions, would liave been but a small fraction over one hour. This period, 

 it is true, would liave been somewhat modified by the counteracting influence 

 of the solar tides; but the hypothesis referred to seems wholly inadequate to 

 meet the objection derived from equation (2). 



