]^]^2 RESEAKCHES CONCERNING THE PERIODICAL 



These circumstances, and the known fact that, in the different systems, in- 

 feriority of size and mass is generally connected with inferiority of perihelion 

 distance, afford a double analogy for concluding a priori that the perihelia 

 of these minute asteroids are gradually condensed as the distance from the 

 sun diminishes, although the law of their aggregation in space will doubtless 

 remain always unknown for want of data for its determination. Since countless 

 millions of these bodies are annually encountered in the small portion of planetary 

 space with which the earth comes in contact, we are led to the inference that 

 the number of the perihelia of these bodies inferior to that of Venus, or even 

 Mercury, is inconceivably great. Indeed, this would, be the case, if these 

 bodies were there scattered as sparsely as in the regions traversed by the earth. 

 But the analogies I have mentioned strengthen the probability that no such 

 rarity prevails within the limits of Mercury's, or Venus' mean distance; and 

 this conclusion once arrived at, a new analogy comes in for its support. I 

 allude to tho extremely interesting discussion between Bessel and Encke, in 

 Schumacher's Astr. Nachr., Nos. 289, 305, and 310, in which the former ob- 

 jects to the hypothesis of a resisting medium, from the fact that its existence 

 is indicated by no other phenomenon in nature. The reply of the latter is, 

 that no other phenomenon in nature is capable of its indication. Encke's 

 comet is only found to be resisted while within the distance of Venus. Now 

 as Halley's comet never goes far within the limit at which this resistance is 

 sensible, and Biela's comet never approaches this limit, the perihelion distance 

 of the former being superior to Mercury, and that of the latter little inferior to 

 that of the earth, neither of these bodies can afford a contradiction or confirm 

 mation. The planets Mercury and Venus, the one within this limit, and the 

 other on its border, are too dense and massive, compared with Encke's comet, 

 to enable us to detect such a resistance. It is with deference to the opinion of 

 these distinguished authors that I venture to suggest the possibility of such an 

 aggregation of these small asteroids within the mean distance of Mercury, as 

 may produce a sensible effect in resisting the progress of a body so light as 

 Encke's comet, which Sir John F. W. HerscheP^ supposes to be incomparably 



^^ Herschel's Astronomy, Chap. x. 303. The author remarks, " that the most unsubstantial 

 clouds which float in the highest regions of our atmosphere, and seem at sunset to be drenched in 

 light, and to glow throughout their whole depth as if by actual ignition, must be looked upon as 

 dense and massive bodies, compared with the filmy and almost spiritual texture of a comet," 



