166 On the Meteors of 13th November, 1833. 
plane of the ecliptic, and having its aphelion near to the orbit of the 
earth. 
Let us inquire, next, what is the periodical time? Since the same 
phenomenon was exhibited at Mocha, on the morning of the 13th 
November, 1832, and on a much larger scale than that, in various 
parts of the world, on the morning of the 12th November, 1799, we 
cannot suppose such a coincidence in the time of the year to have 
been purely accidental, but must conclude that the periodical time of 
the comet, and that of the earth, bear to each other a ratio which can 
be expressed in whole numbers ; so that after a certain number of 
revolutions of the two bodies, corresponding to the terms that ex- 
press their ratio, they will come together again. ‘They could not 
come together, as they did, on two successive years, unless the peri- 
odical time of the comet was nearly an aliquot part of that of the 
earth, such as one half, one third, &c. Now, if the time be any ali- 
quot part of a year, it must be one half, so that the comet would per- 
form two revolutions, while the earth performs one ; for, were its pe- 
riod only one third of a year, the line of the apsides would not be long 
enough to reach the earth. This will be obvious from the following 
estimate. Let D represent the axis major of the earth, and d that of 
the comet’s orbit, their times being as 3to 1. Then, by Kepler’s 
Pay, weeks ih? 4 yD) ds: 
Taking D=190,000,000 miles, d=91,343,000 for the whole ma- 
jor axis, which is not equal to the distance from the sun to the earth. 
But, supposing the times as 2 to 1, we have 
22 :12::D? : d%, whence d2=119,692,000 miles ; giving for the 
perihelion distance 24,692,000, and for the aphelion 95,000,000 
miles. Hence we conclude, (3.) that the body has a period of near- 
ly six months, and its perthelion a little below the orbit of Mercury. 
The transverse axis and the foci being determined, the ellipse 
may be described. Therefore, join CS, and produce the line CS 
to D, making SD equal to the perihelion distance, and upon CD de- 
scribe the ellipse CFD, and it will represent the orbit of the comet. 
This is to be regarded only as a first approximation to the true 
periodic time. The distance from the sun, instead of being taken, as 
here, at the extremity of the body, ought to be reckoned from the 
center of gravity, if we knew where to fix that. Nor can we sup- 
pose that the periodical time is very uniform, since a light nebulous 
body like the one in question, crossing as it does the orbits of Venus 
and Mercury, and having its perihelion near the orbit of the latter, 
