50 



REPORT 1871. 



In the following Table Professor Kirkwood compares together the aphelion 

 distances of the several known comets of short periods with the mean dis- 

 tances of the several larger planets from the sun : — 



2 a 



Comets. 



Encke's . . . 

 1S19, IV.... 

 De Vice's. . . 

 Pigott's "1 



(1783)/ 

 1867,11.... 



1743. 1 



1766,11. ... 



8. :i8i9, III... 



9. Brorsen's . 



10. D'Arrest's , 



11. Faye's 



12. Biela's 





409 

 4-81 



5'02 



5-28 



5-29 

 5-32 



5 '47 

 5-55 

 5-64 



575 

 5"93 

 619, 



2. 



P-1 o 



-g 



^ a 



«.2 





1. 

 2. 

 3. 

 4. 



5. 



6. 



Comets. 



Peter's (1846, VI.). 

 Tattles (1858, I.) . 





9 '45 

 10-42 



Saturns's mean 

 distance 9' 54. 



1. 1867,1 



2. November Meteors. 



1866,1. 



i9'28 

 1965 

 19-92 



Westphal's (1852, IV.) 



Pons' (1812) 



Gibers' ('1815) 



|De Vice's (1846,1V.).. 

 Brorsen's (1847, V.) .. 

 Halley's 



31-97 

 33-41 

 34'o5 

 34-35 

 35-07 



35-37 



Uranus's mean 

 distance 191 8. 



Neptune's mean 

 distance 30-04. 



It is also evident that the passage of the solar system through a region of 

 space comparatively destitute of cometie clusters would be indicated by a 

 corresponding paucity of comets. Such variations of frequency are, indeed, 

 found not only in the records of comets, but also of meteoric showers which 

 have been accidentally recorded, the greater number of the latter having 

 been observed during the five centuries between TOO a.d. and 1200 a.d., and 

 again in those following a.d. 1700, suggesting that dm-ing the former and, 

 perhaps, again during the present period the solar system is passing through 

 a cosmical or meteoric cloud of very great extent, — not less, indeed, on the 

 received speed of the sun's proper motion, than fourteen times the -width of 

 Neptune's orbit. Professor Kirkwood adds, in particular reference to the 

 August meteor-system, " The fact that the Aiigust meteors, which have been 

 so often subsequently observed, were first noticed in 811 [see M. Quetelet's 

 Catalogue of Star-showers] renders it probable that the cluster was intro- 

 duced into the planetary system not long pre-viously to the year 800. It may 

 be also worthy of remark that the elements of the comet of 770 a.d. are not 

 very different from those of the August meteors and of the third comet of 

 1682 " *. With regard to the sun's passage through 

 a meteoric cloud of the above-considered dimen- 

 sions and constitution it is noticed that the num- 

 ber of coraetary perihelia found in the two qua- 

 drants of longitude towards and from which the sun 

 is moving is 159, or 62 per cent., and that of peri- 

 helia in the two other quadrants is 98, or .38 per 

 cent., shoAving their tendency to crowd together 

 about the direction of the sun's proper motion in space. The large excess of 



260^ 



* The interval bet-ween tbe perihelion passage of 770 and that of 1862 is equal to 9 

 periods of 121-36 years. Oppolzer's determination of the period of 1862, III., is 121-5 

 years. Hind remarks that the elements of the Comet of 770 are " rather uncertain," but 

 says " that the general character of the orbit is decided." It may be worthy of remari 

 that a great meteoric sbo-wer, the exact date of which has not been preserved, occurred ir 

 770. 



