OBSERVATIONS OF LUMINOUS METEORS. 231 



distance (or about ten times the moon's distance) above the earth's orbit. 

 Thus the observation of a star-shower not far from the latter date, between 

 the 7th and 19th of August (or on August 10th, S. & Z. 140, in the above 

 comparative list), with a radiant-point corresponding closely to that of meteor 

 joourstiivants of the comet at this point, is in satisfactory correspondence with 

 the earth's conjunction with this comet's orbit, although the date of the 

 shower and of the nearest conjunction of the orbits is not exactly that of the 

 earth's passage across the line of the comet's nodes. In several other cases 

 (as in that of Lexell's comet) of comets moving nearly in the ecliptic, the 

 point of nearest conjunction and the time of the year when the earth passes 

 through it are very far removed from the place and from the corresponding 

 time of the earth's passage through the node ; and the approach of the two 

 orbits is yet often closer at the former than at the latter place. 



The particulars of each comet's approach to the earth, whether occurring at 

 the node or appulse, will be found, as thus described, in the columns of the 

 accompanying lists, the dates in column 3 and the places of the radiant-points 

 in columns 5 and 6 being brought up (for precession) to the year 1875, neg- 

 lecting any perturbations which the orbit of the comet since the time of its 

 appearance may have undergone. In cases of appulses (or of earth's conjunction 

 with the comet-orbits at a common radial distance from the sun), the motions 

 of the meteor-particles are supposed to be equal and parallel to that of the 

 comet ill its orbit there, or at the point where its radius vector is equal to 

 the earth's distance from the sun ; as no regard is paid in the graphical 

 construction to the slightly elliptic form, both of the orbits of certain comets 

 and of the earth's orbit, which are severally assumed to be parabolic and 

 circular, small errors on these accounts will present themselves in the lists, 

 which, for preliminary purposes, may be looked upon as unimportant. 



A -f or — sign following the dates indicates if the comet's motion is direct 

 or retrograde ; and if the comet was approaching the sun, or if it was very 

 near to its perihelion at the node or appulse, there is added after the radius 

 vector, or appulse-distance, in column 4, a notation sign (* or §) denoting 

 these conditions ; where no such sign is added, the comet's motion is receding 

 from the sun. The italic letters after the comets' years in column 2 are 

 intended to supply some information of their general characters and appearance. 

 Thus d implies just discernible by the naked eye, d plainly, and D brightly so ; 

 and D a comet visible by day. 1 1 TT indicate corresponding proportions in 

 the apparent dimensions of the tails: t less than 5°; t, 5° to 15°; T, 15° to 

 30° ; and T upwards of 30° in length. Durations of the comets' periods, 

 where elliptic orbits are known to belong or have been calculated and assigned 

 to them, are also roughly indicated by letters corresponding to their lengths 

 of period thus : I, periods less than 15 years ; 1, 15 to 50 years ; L 50 to 400 

 years ; and L comets of very long periods exceeding 400 years. The letter 

 p is added to comets having decidedly parabolic orbits, and h to those whose 

 orbits are computed to have been hyperbolic. The sign || following these cha- 

 racters indicates that at its appearance the comet passed very near the earth. 

 The initials P., H. affixed to the comet of 1490 (N. 2) are those of two inde- 

 pendent computers, Peirce and Hind, of two distinct and apparently equally 

 probable orbits of the comet ; while the mean of two independent sets of orbit- 

 elements assigned to it by Pingre is adopted in the Table for the comet of 



P . 



1582 (N. 83), to whose designation a similar initial „ is affixed. The orbit- 

 elements used in the rest of the Table are those of Hind's work ' The Comets,' 



