320 -P^^. Olmsted on the Zodiacal Light. 



phenomena of shootirjg stars. As the leading steps hy which I 



arrived at this conclusion, after an extensive induction of facts, 

 were very brief and simple, I may be permitted to repeat them 

 here. I argued thus: If all the meteors which fell on this 

 occasion (which were in vast numbers, and some of them proved 

 to be bodies of comparatively large size,) had been restored 

 to their original position in space, they would of themselves 

 have composed a nebulous body of considerable extent. But, 

 since the same shower had been several times repeated without 

 any apparent exhaustion of the nebulous body, it was inferred 

 that only small portions of that body came down to us, such 

 as constituted its extreme parts which approximated nearest to 

 the earth ; and various reasons induced the belief that the nebu- 

 lous body itself was one of very great extent. It was a striking 

 fact that the earth had, during several preceding years, fallen in 

 with this body at exactly the same part of its orbit. Now, since 

 it is impossible to suppose that a body thus situated, and conse- 

 quently subject to the sun's attraction, could have remained at rest 

 in that part of the earth's orbit while the earth was making its 

 revolution around the sun, the conclusion was that the nebulous 

 body itself has a revolution around the sun, and a period of its 

 own. Since the earth and the body met for several successive 

 years at the same point of the ecliptic, that period must obviously 

 be either a year or less than a year. It could not be more than 

 a year, for, in that case, the body would not have completed 

 its revolution so as to meet the earth at the same point for suc- 

 cessive years. Its period might be a year, and it might be less 

 than a year provided the time was some aliquot part of a year, so 

 as to make it revolve just twice or three times, &/C., while the 

 earth revolves once. The time being given we easily find the 

 major axis of the orbit by Kepler's third law. On trying so short 

 a period as one third of a year, it gives a major axis too short to 

 reach from the sun to the earth, and hence it was inferred that 

 the body could not have so short a period as four months, since it 

 would never in that case reach the earth's orbit, even at its aphel- 

 ion. A period of six months was found to be sufficient, and this 

 Avas accordingly assumed at first to be the time, although the pos- 

 sibility that the period might be a year was distinctly admitted. 

 But, extensive as I even then believed the nebulous body to be, 

 I had formed very inadequate notions of its real extent, for this 

 may clearly be sufficient to reach from the sun to the earth, and 

 thus to correspond in dimensions to the zodiacal light ; and since 

 the center of gravity of this body may be far within the earth's 

 orbit, so its orbit may, even at its aphelion, be distant from the 

 earth, and yet the extreme portions of the body may reach beyond 

 the ecliptic. It would, therefore, be entirely consistent with my 

 original views, to assign to a nebulous body of such an extent as 





