Zodiacal Light. 163 



successive 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 aphelion. A 

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

 was accordingly assumed at first to be the time, although 

 the possibility 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 dimen- 

 sions to the zodiacal light ; and since the centre 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 that of the zodiacal light, a period as short as one- 

 third of a year or even less. 



I do not assert positively that the zodiacal light is the veri- 

 table body which produces the meteoric showers of November 

 and August. Before such an hypothesis can be proved to be 

 true or false, with certainty, a greater number of precise 

 observations, continued through a series of years, would re- 

 quire to be made, and a careful comparison instituted between 

 the hypothesis and the facts. Should the zodiacal light be 

 found at last incompetent to explain the periodical meteors, 

 the existence of a nebulous body, as inferred from a full 

 survey of the facts in the case of the meteoric shower of 

 November 13, 1833, independently of all hypothesis, will still 

 be true. But, with great deference, I submit to the Asso- 

 ciation the following presumptions in favour of the opinion 

 that the zodiacal light is the nebulous body which produces 

 the meteoric showers of November. 



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