1889.] of the various Species of Heavenly Bodies. 203 



band given at wave-length 558 being evidently due to the radiation 

 of the latter element, since the band fades away in both directions. 



Another band was measured at 467*9, and is most probably the 

 carbon band at 474 which under certain conditions has it maximum 

 at 468 instead of 474. 



On May 6th the comet was again observed. A very faint line was 

 seen at 569 and another at 543. These were probably due to the lead 

 flutings at wave-lengths 568 and 546. 



The apparent absence of lead in the spectrum observed on May 5th 

 may probably be due to the incompleteness of the observations on 

 that date in comparison with those made on May 6th. Or it may be 

 that the greater brightness of the continuous spectrum masked the 

 two faint remnants of the lead fluting. 



Other bands were observed on May 6th, the hot carbon and the 

 manganese radiation at 558 being clearly indicated. 



An observation was made on May 15th at Greenwich* and it is 

 interesting to note the change that had taken place. A band at 517 

 was measured, and two others observed, one about 483 and another 

 about 561. Here, clearly, we have indications of cool carbon radia- 

 tion occurring as the comet receded from the sun, the observations 

 having been made nearly a month after perihelion. 



As the comet receded from the sun, then, manganese absorption 

 was succeeded by manganese radiation, hot carbon being indicated in 

 both cases. No further observations were made until nine days after 

 the latter condition was observed, and then the spectrum was that of 

 cool carbon. Doubtless there was an intermediate stage in which 

 hot carbon was observed alone. 



IX. POSSIBLE CAUSES OF COLLISIONS IN COMETS. 



Internal Work. 



Professor Tait's view as to the origin of collisions in a meteor-swarm 

 entering our system as a comet was that they were a consequence of 

 the movement of the individual meteorites along approximately 

 elliptic orbits, described in something like equal periods in any plane 

 about their common centre of inertia. 



The group was also supposed to be subjected to a sort of tidal dis- 

 turbance by the sun.f 



It is certain that one of the principal causes of the increase of 

 temperature of a comet during its approach to perihelion is the 

 increased number of collisions due to the greater tidal action which 

 takes place. Hence the larger the swarm, the greater the difference 

 between the attractions of the sun upon opposite sides of it, and there- 

 fore the greater the disturbance set up. Also, the shorter the 

 * ' Greenwich Observations,' 1877. 

 f ' Edinb. Eoy. Soc. Proc.,' vol. 10, p. 367, 1879. 



