Klrkwood.] ilU [Nov. 21, 



gaseous, or even clusters of solid meteorites, the difference between the 

 sun's attraction on the central and the superficial parts would have pulled 

 the comets asunder, spreading out the fragments into somewhat different 

 orbits, like the meteoric streams of August and November. 



This view of the gradual dispersion of comets in perihelion is in striking 

 harmony with the facts of observation. The comets of short period have 

 not only been divested of tails, which in all probability they originally 

 possessed, but they seem to be losing more and more of the cloud-like 

 matter which surrounds their nuclei. Halley's comet has lost much of its 

 ancient splendor, and. had its period been no greater than that of Encke's 

 or Biela's, it might long since have been reduced to a telescopic magni- 

 tude. The separation of Biela's comet, in 1845, was not the beginning of 

 that body's dismemberment. We have evidence that this process had 

 commenced before 1798, as in that year a meteoric shower, produced by 

 its debris, was observed in Europe. A shower derived from the same 

 group was again seen in 1838.* Before 1845, however, the separated frag- 

 ments were too small to be individually recognized. How far the sun's 

 action alone can explain the facts, it may be impossible to determine. 



VI. The Zodiacal Light. — Original small planets near the sun, in a 

 nebular or gaseous condition, would probably be transformed either into 

 rings or meteoric clusters, the scattered particles of which, reflecting the 

 sun's rays, would present an appearance like that of the zodiacal light. 



VII. Origin of the Asteroids. — In the primitive condition of a 

 planet, immediately after its separation from the central mass, not only 

 would tlie latter cause a considerable elongation of the former in the direc- 

 tion of the line joining their centres, but the planets also— especially tlie 

 larger — would produce great tidal elevations on the sun's surface. Now, 

 a comparison of the elements of Hilda and Ismene, the 153d and 190th 

 asteroids, shows them to be an isolated pair whose periods are very nearly 

 equal, each exceeding the longest in the interior cluster by more than fif- 

 teen months. Jupiter's limit of equilibrium, when in the nebular form, 

 was immediately beyond the orbits of these minor planets. If the sun 

 once extended to the aphelion distance of Hilda (4.632), the central attrac- 

 tion of his mass on a particle of the equatorial surface was but five times 

 that of Jupiter at the point to which he was vertical. f The centrifugal 

 force due to the sun's rotation would be greatest at the crest of this tidal 

 wave, produced by Jupiter, so that parts might become separated from the 

 solar mass, and transformed into asteroids. It is to be further remarked 

 that two periods of Jupiter are approximately equal to three of Hilda and 

 Ismene, that is, to three rotation periods of the sun at the epoch of their 

 separation. The disturbing effect of the "giant planet" on the tides of 

 the central body would therefore be increased at each perihelion passage. | 



* Humboldt's Cosmos, Bohn's ed.. Vol. iv, p. 582. 



+ Jupiter's perihelion distance is -1.95. 



X The longitude of Ismene's perihelion diflers from that of Hilda's by 180°. 



