Kirkwood.J ^io [Nov. 19, 



intei-sect the earth's path were so changed by perturbation towards the 

 close of the 12th century as to prevent the appulse of the meteoric groups 

 with the earth's atmosphere ; or, (2) the nebulous matter is very unequally 

 diffused thi'ough the sidereal spaces. That the former has not been the 

 principal cause is rendered extremely probable by the fact that the num- 

 ber of epochs of periodical showers was no greater during the cycle of 

 abundance than in that of paucity. We conclude, therefore, that during 

 the interval from 700 to 1200 the solar system was passing through, or 

 near, a meteoric cloud of very great extent ; that from 1200 to 1700 it was 

 traversing a region comparatively destitute of such matter ; and that 

 about the commencement of the 18th century it again entered a similar 

 nebula of unknown extent. 



The fact that the x^ugust meteors, which have been so often subse- 

 quently observed, were Ji)'st noticed in 811, renders it probable that the 

 cluster was introduced into the planetary system not long previous 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 the 3d comet of 1862. - 



Adopting Struve's estimate of the sun's orbital velocity, we find the 

 diameter of the nebula traversed in 500 years to be 14 times that of 

 Neptune's orbit. 



It is remarkable that with the exception of Mars the perihelia of the 

 orbits of all the principal planets fall in the same semi-circle of lon- 

 gitude — a fact which can hardly be regarded as accidental. Now, if the 

 orbits were orginally circular, the motion of the solar system through a 

 nebulous luass not of uniform density would have the obvious effect of 

 compelling the planets to deviate from their primitive orbits and move in 

 ellipses of various eccentricities. It is easy to perceive, moreover, that 

 the original perihelion points of all the orbits would be on that side of 

 the system which had passed through the rarer portion of the nebulous 

 mass. We have thus a possible cause of the eccentricity of the planetary 

 orbits, as well as of the observed distribution of their perihelia. f 



4. The particles of a cometic mass, being at unequal distances from 

 the sun, will tend to move at different rates and in somewhat different 

 orbits. This tendency will gradually overcome the feeble attractive force 

 between the particles themselves. The most distant parts will thus 

 become separated from the nucleus, and move in indeiDendent orbits. 

 The motion of such meteoric matter will be in the same jjlane with that 

 of the parent comet ; the orbit of tlie former, however, being generally 

 exterior to that of the latter. The connection recently discovered be- 

 tween comets and meteors, and especially the fact that the period of the 



* The interval between the perihelion passage of 770 and that of 1862 is equal to 9 periods of 121.35 

 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 remarlt that a great meteoric shower, the exact date of 

 which has not been preserved, occurred in 770. 



t This suggestion is due to R. A. Proctor, F. R. A. S., the distinguished author of "Saturn and its 

 System." 



