CERTAIN HARMONIES OF THE SOLAR SYSTEM. 67 



2. It will account for the phenomena common to all appearances of the zodiacal 

 light, broad base and all. 



3. It accounts for certain periodical changes in form and intensity, etc., of the 

 same, which seem to be completed in a synodical revolution of the moon. 



4. It provides for the gegenschein in form and position ; and possibly also for 

 "a lunar zodiacal light." 



5. It renders a plausible account of the fading, at times, and total disappearance 

 of the Zodiacal Light. 



6. It accounts for the absence of a determinate parallax of the girdle. 



7. It shows why, when east and west zodiacal lights are visible at the same 

 time, the middle, even, of the zodiacal arch need not be wholly obscured by the 

 earth's shadow. 



8. It provides for the "pulsations." 



Origin of the Girdle. 



(96) It remains to consider how far the origin of the girdle may be accounted 

 for by the modified nebular hypothesis, already so frequently applied. 



If the moon herself were formed of a spheroidal shell [such as those described 

 in (37)], while the form of the earth with its expanded atmosphere was yet very 

 oblate; the equatorial diameter extending beyond the present distance of the 

 moon — i. e. more than 60 times the radius of the earth's equator — the moon, 

 derived from the atmosphere of this spheroid, might, at first, indeed have had the 

 form of a spheroidal shell, with its equatorial circle nearly in the ijlane oj the ecliptic, 

 as the orbit of the moon now is, instead of the plane of the eartlis eqitator, since 

 determined. 



This whole collection of material having, by processes heretofore described, (26), 

 been brought to revolve together, the outer portions having thereafter failed to be 

 collected with those that went to form the moon herself, these same outer portions 

 would still continue to revolve and complete tlie same periodic time. 



The part between the moon and the earth would nearly all be compelled to fall 

 toward the earth in obedience to her superior attraction; except, possibly, some 

 small remnant still forming an extra-mundane nebulosity (the middle of it at the 

 position A in Fig. 13); the existence of which might help to account for some of 

 the phenomena of solar eclipses, if not also of those of transits of the inferior 

 planets ; which it would be out of place to enlarge upon in this connexion.^ 



(97) Whether the material which exhibits the Aurora Bar ealis, or rather Aurora 

 Polaris, can have had a similar origin, near to the pole of the oblate expanded 

 atmosphere, and so, also, near to the pole of the Ecliptic in direction, as well as 

 actually near to the earth, can be little better than matter of conjecture. The 

 results, of the spectrum-analysis [(74) and Note'\ do not yet establish a composition 



' The present Astronomer Royal, Sir George B. Airy, is understood to have said, soon after the 

 total eclipse of the snn, in 1842, that some of the phenomena of that eclipse required for their expla- 

 nation the supposition of the existence of a material between the moon and the earth. 



