INTRODUCTION. XXVII 



requires in all the parts of a ring, and in their cooling, ought to make this phenomenon very 

 rare. Thus, the solar system presents hut one example of it that of the rings of Saturn. 

 Almost always each ring of vapor ought to be broken into several masses, which, carried on 

 with velocities differing little from each other, would continue to circulate at the same distance 

 around the sun. These masses ought to take a spheroidal form, with a movement of rotation 

 in the direction of the rotation, since the inferior molecules have a motion less than the supe- 

 rior ones; they have thus formed so many planets in a state of vapor. But, if one of them has 

 been sufficiently powerful to unite, successively, by its attraction, all the others around its centre, 

 the ring of vapor will have thus become transformed into only one spheroidal mass of vapors, 

 circulating around the sun with a rotation in the same direction as its revolution. Now, if 

 we follow the changes which further cooling ought to produce in the vapory planets, of whose 

 formation we have just spoken, we shall see grow up in the centre of each of them, a nucleus, 

 incessantly increasing by the condensation of the atmosphere which surrounds it. In this con- 

 dition, the planet resembles perfectly the sun in the nebulous state, in which we have just been 

 considering it ; the cooling ought, then, to produce., at different limits of its atmosphere, 

 phenomena similar to those we have described ; namely, rings and satellites circulating around 

 its centre, in the direction of its motion of rotation, and turning on their axes in the same 

 direction." Systeme du Monde. Paris edition: pp. 467-468. 



This great theory of Laplace, called his nebular theory, appears to be looked upon by astron- 

 omers with wonder, almost with awe, and as a thing which they may scarcely dare to touch. It 

 is regarded with favor, yet there are few cosmologists who venture a decided opinion upon it ; 

 and, indeed, while there are few points from which it can be controverted, Laplace himself 

 seems to have exhausted what can be said in its favor, in the few lines which he has given to it, 

 in a manner far from positive, and in a retired corner of his book. If that theory be true, however, 

 we have reason to think that no one of the planets may have absorbed in itself all the nebulous 

 matter of the ring from which it was originally formed ; and that, consequently, there may be, 

 to each of them, a remainder substance, in the form of a ring, or rings, with the planet for its 

 centre. In the case of Saturn, such rings are visible by the aid of our glasses. To Jupiter, 

 such rings have given four satellites ; for our own globe, one satellite has been produced. And 

 we may well query, whether there may not be still a remainder of the nebulous matter left from 

 the ring originally producing the earth ; the nebulous substance of that ring not having been 

 all exhausted in the formation of our earth and its moon, and showing itself in a ring such as 

 we are now considering. 



But, avoiding any consideration of these topics, as regards other planets, and confining our- 

 selves simply to the facts of the Zodiacal Light, and of a ring central to the earth, to which 

 they seem to lead us, we proceed to apply the results of Bouguer's experiments to this case. 



In the annexed diagram, constructed according to this supposition,* the observation quoted 

 in the former sections that of September 4th, 1853 is again taken as an example, and for the 

 same reason ; namely, that it is a simple one, and one also in which the spectator is near the 

 plane of the ecliptic. The horizons, at 4 h 30'", 3 h 30 m , 2" 30 m , l h 30 m , and at midnight, are 

 given, together with the lines of the spectator's vertices, as well as his positions, o o, &c., at 

 4" 30 m and 3" 30 m , &c. A B F C are the boundaries of the Zodiacal Light at 4" 30 m , and 



The relative proportions of the earth and the ring, and also its distance, are, of jourse, not given in this diagram 

 with any effort at certainty ; the upward extent of the ring is probably far greater than can he here represented. The 

 diagram is, however, sufficiently correct for our present purposes of elucidation. 



