between Comets and Meteors. 189 



gether by the previous repetitions of this process*. In this way, 

 a comet while moving in outer space, beyond the reach of the 

 many powerful disturbing influences which prevail within the 

 solar system, would inevitably accumulate within itself just 

 such a globular cluster of meteorites as the November meteors 

 must have been before they became associated with the solar 

 system. 



When this body of meteors, enveloped by their comet, swept 

 past the planet Uranus in the year 126, they may have come so 

 close that the comet brushed against the atmosphere of the planet. 

 If this took place, the comet must have both received a motion 

 of rotation and been retarded f. The meteorites at its centre re- 

 taining their speed would accordingly gradually pass out through 

 it and leave it a little behind ; and when all got so far from the 

 planet as to be beyond its further influence, the comet would be 

 found moving round the sun with a shorter periodic time than 

 the meteors. This is in conformity with Dr. Qppolzer's deter- 

 mination of the periodic time of the comet, viz. 33*18 years, 

 that of the meteors being 3325 J. 



* The behaviour described in the text is a consequence of the familiar 

 formula for elliptic motion 



r a 



since, if at any distance a resistance r be experienced, V is thereby dimi- 

 nished, and, as the formula must still hold good, a is also shortened. 



t The cluster appears to have approached the orbit of Uranus from the 

 outside, and, after passing the planet, to have described a relative orbit 

 directed a little inwards towards the sun, but principally backwards, i. e. 

 in a direction the reverse of the planet's motion, with a relative velocity 

 greater than the velocity of the planet. It in this way acquired a slow ab- 

 solute motion, which was directed both inwards and backwards, and was 

 thus started in its retrograde orbit round the sun. A slight brush of the 

 comet against the planet would both somewhat increase the curvature of 

 the relative orbit, and slacken the comet's pace along it ; and either of these 

 effects would, under the circumstances which have been described, result in 

 such a diminished absolute velocity as is attributed to the comet in the text. 



X It should be remarked, however, that the comet seems to have fallen 

 nearly a revolution behind the meteors since a.d. 126, i. e. in 52^ revolu- 

 tions. If this be so, its periodic time must be less than Dr. Oppolzer's 

 estimate, and is probably about 32'63 years — unless we may suppose that 

 since its introduction into the solar system it has suffered a perturbation 

 which has diminished its mean motion round the sun. Such a perturbation 

 is not impossible ; it would arise, for instance, if a swifter stream of meteors 

 overtook the comet and passed through it ; and it is easy to assure oneself 

 that a swarm of meteors having the requisite direction and speed to behave 

 thus may have been drawn into the solar system by any one of the planets 

 Jupiter, Saturn, Uranus, or Neptune. 



It is not without interest to observe that whether the periodic time of the 

 comet be 33'18 years or less, it will, before its next perihelion passage, have 

 been run into by the meteors. The effect of this would seem to be first to 



