ISoO.] 



217 



[Kirkwood. 



150 years, from 1 GOO to 1750, 16 comets were visible to tlie naked eye;* 

 of wliicli 8 appeared in the 25 years from 1664 to 1689. Again, during 

 60 years, from 1750 to 1810, only 5 comets were visible to the naked eye, 

 while in the next 50 years there were double that number. The probable 

 cause of such variations is sufficiently obvious. As the sun in his pro- 

 progressive motion approaches a cometary group, the latter must, by 

 reason of his attraction, move toward the centre of our system, the nearer 

 members with greater velocity than the more remote. Those of the same 

 cluster would enter the solar domain at periods not very distant from each 

 other ; the forms of their orbits depending upon their original relative 

 positions with reference to the sun's course, and also on planetary per- 

 turbation. It is evident also that the passage of the solar system through 

 a region of space comparatively destitute of cometic clusters would be 

 indicated by a corresponding paucity of comets. By the examination, 

 moreover, of any complete table of falling stars we shall find a still more 

 marked variation in the frequency of meteoric showers. 



Previous to 1833, the periodicity of shooting stars had not been sus- 

 pected. Hence the showers seen up to that date were observed accu 

 dentally. Since the great display of that year, however, they have been 

 regularly looked for, especially at the November and August epochs. 

 Consequently the numbers recently observed cannot properly be com- 

 pared with those of former periods. Now, according to the Catalogue of 

 Quetelet, 244 meteoric showers were observed from the Christian era to 

 1833. These were distributed as follows : 



A remarkable secular variation in the number of showers is obvious 

 from the foregoing table. During the 5 centuries from 700 to 1200, 110 

 displays are recorded ; while in the 5 succeeding, from 1200 to 1700, tlic 

 number is only 25. It will also be observed that another period of abun- 

 dance commenced with the 18th century. A catalogue of meteoric stone- 

 falls indicates also a corresponding increase in the number of serolites, 

 which cannot be wholly accounted for by the increased number of ob- 

 servers. Now, there are two obvious methods by which these variations 

 may be explained. Either (1) the orbits of the meteoric rings which 



*See Humbol(ifs Cosmos, vol. IV. p. 

 since as 18(51. 



.WS, The writer called attention to this variation as Ions 



