20 



by a great multitude of these small meteors or asteroids, though their 

 effect is insensible on Mercury and the other primaries, owing to their 

 superior mass and density, and as Enckc remarks, also insensible on 

 Halley's and Bicla's comets, whose perihelion distances, respectively, 

 correspond nearly with those of Venus and the earth. It is only ne- 

 cessary to suppose that in some planes these bodies exhibit a greater 

 tendency to the formation of clusters, or possibly of flattened rings, 

 in order to account for anniversary periods of remarkable showers; 

 since the earth revisiting the same plane at the same season of the 

 year, and at the same distance from the sun, may or may not en- 

 counter one of these clusters or parts of a flattened ring. But these 

 clusters continuing to move in the same plane, the earth must, if it 

 meet them at all, do so at anniversaiy periods. On the supposition 

 of a flattened ring, the node having the same radius vector as the 

 earth, these displays might occur for several anniversaries, and then 

 cease for an indefinite period, owing to the motion of the apsides 

 of the ring; till the anomaly which has a radius vector equal to the 

 earth's mean distance, again coincides with one of the nodes of the 

 ring. Hence the connexion between the periods of the second table, 

 as far as regards our knowledge of them is accidental, since they de- 

 pend not on the orbital period of these bodies round the sun, but on 

 the circumstance of the earth's encountering one of these clusters, 

 or planes abounding in them, which is regulated by a law of distribu- 

 tion of these bodies in planetary space, that must always remain 

 unknown, for want of data for its determination. 



The author conjectures that the meteors termed sporadic, by Que- 

 telet, which have no common convergent point, may have their peri- 

 helia superior to those of the periodical meteors, and their aphelia far 

 superior to that of the earth. In such a case, their orbital velocity 

 would be as great as that of the earth, or greater ; and as they move 

 in all varieties of direction, the earth's tangential motion does not 

 cause them to tend, relatively towards a convergent point, in nearly 

 an opposite direction, as it does with meteors moving very slowly in 

 their orbits, whatever may be their true directions in space. 



A brief history of the opinions and theories of writers on this sub 

 ject is given ; and an oversight pointed out in Professor Erman's paper 

 quoted by the author in an oral communication of August 21st, 1840 

 This relates to Prof. Erman's minimum relative velocity of the me 

 teors, which, instead of being 0.83, of that of the earth, may be inde 

 finitely small, and therefore, in his formulae [Astronomische Nachrich 

 ten, No. 385, p. 9,] may give a motion of the convergent point indefi 



