340 Mr. G. Johnstone Stoney [Feb. 19, 



since become sporadic Leonids. They traverse new ortits a little 

 differing from the great meteoric orbit, and also differing from one 

 another. By a well-known dynamical law, they would, if subsequently 

 acted on only by the sun's attraction, return accurately at the end of 

 each revolution to the situation close to the earth's path which they 

 occupied when the earth, after having dragged them a little aside, 

 passed on along its own orbit. Since the sun's attraction upon them 

 is immensely more powerful than any other, they, on the completion 

 of every revolution, return nearly to that situation, which the earth 

 passes each year in the middle of November ; but since their motion^ 

 are slightly perturbed, especially by the great planets Juj)iter and 

 Saturn, tbey get to| be somewhat scattered into situations behind and 

 in front of that point in the earth's orbit, as well as, no doubt, many 

 of them sideways, so that a few of them may encounter, though many 

 more of them must escape, the earth. This scattering of the sporadic 

 Leonids is what causes the earth to meet with a few of them for 

 some days before and after it reaches the point of intersection of its 

 orbit with that of the main swarm. 



Again, when the earth diverts a meteor from its path, it slightly 

 alters every element of its orbit. Among others, it alters its periodic 

 time. Hence in each subsequent revolution the meteor which has 

 been disturbed will either draw ahead of the main swarm or fall 

 behind it ; and this has caused the sporadic meteors to be now dis- 

 tributed round the whole length of the orbit, so that the earth 

 encounters some of them every year, and not only at intervals of 

 33 years. 



Such is a sufficient picture of what happens in the case of ordi- 

 nary sporadic Leonids. But there is one among them which is go 

 peculiar that it deserves separate treatment. 



Of TempeVs Comet. 



Astronomers know very little of the dynamics of comets, very- 

 little of the dynamics of clusters of stars, and almost nothing of the 

 dynamics of nebulae. When any one of these problems shall be 

 solved, it will probably throw much light on the other two. Mean- 

 while, whatever may be the dynamical relation in which the tail of a 

 comet stands to its nucleus and to the other bodies of the solar 

 system, we know at all events that its nucleus travels along an orbit 

 Under the same laws as an ordinary mass of ponderable matter. Now 

 the orbit of the nucleus of Tempel's comet is nearly but not quite 

 coincident with that of the main swarm of November meteors, a^ 

 appears from the following table of the best determinations we yet 

 have of the elements of both orbits. 



Leonids. Tempel's Comet. 



Period .. .. .. 33-25 .. 33-18 yenrs- 



Mean distance 10-3402 .. lC-3248 



Excentricity 0-9047 .. 0-9054 



Perihelion distance .. .. .. ., 0-9855 .. 0*9765 



Inclination .. 16° 46' .. 17° 18' 



Longitude of node 51° 28' .. 51° 26' 



Distance of perihelion from node .. 6^51' .. 9 2' 



