3 
68 | SCIENCE. 
struments. The comet. seems to have been 
discovered independently at Paris by Bouvard, 
who describes it thus: ‘‘ Cette cométe était 
trés petite. Elle ne fut visible 4 la simple vue 
que pendant quelques jours. Le 18 aoit son 
noyau, assez brillant, était entouré d’une né- 
bulosité qui offrait l’apparence d’une cheve- 
lure et d’une queue d’environ 2° de longueur.”’ 
Bode reports the comet visible to the naked 
eye on Sept. 9, 1812, and on Sept. 14 he gives 
the tail as 1° long; while on the same date, at 
Seeberg, the tail is given as 2° 17’, and the 
diameter of the nucleus 5.4 
seconds (time). The last 
observation which we find at 
Peon tn this appearance was at Mar- 
seilles on Sept. 27, 1812, the 
comet being then just visible 
in the morning twilight. 
tol From the observations of 
1812, covering a period of not 
PR. 25. 
S MAY 25: 
PATH OF PONS-BROOKS COMET. 
quite ten weeks, several orbits were com- 
puted, that of Encke assigning a period of 
70.68 years. More recently Messrs. Schulhof 
and Bossert, from an exhaustive discussion 
of all the observations available (including 
some not known to Encke), predicted a return 
to perihelion about September, 1884, though 
they pointed out that in their period there was 
an uncertainty of +5 years. The comet was 
actually found by Brooks (Phelps, N.Y.) on 
Sept. 1, 1883, some time before it had reached 
the sweeping ephemeris of Schulhof and 
Bossert ; but its identity was soon established. 
The annexed diagram will assist in forming 
an idea of the path in which the comet is 
moving. The earth’s orbit (the northern side 
uppermost) is shown orthographically pro- 
Ta ¥ 
[Vor. III., No. 50. 
jected upon the plane of the comet’s orbit. 
The data necessary for defining the ellipse in — 
which the comet moves are, the angle Q 
(254°), the longitude of the ascending node ;° 
the angle IT— 8 (—161°), the difference be- 
tween the longitude of the node and the longi- 
tude of perihelion (11) ; the angle 7, the incli- 
nation between the earth’s orbit and that of 
the comet; q, the perihelion distance (0.775) 
expressed in units of the earth’s distance from 
the sun; 7’, the date of perihelion passage ; 
and e, the eccentricity (0.96), or ratio, — 
distance from centre to focus 
semi-axis major 
Q and TI — 9 are shown in the figure ; and, to 
form the complete picture, we are to imagine 
the plane of the comet’s orbit revolved about 
the line A B, the line of nodes, until it makes 
an angle of 74° (7) with the plane of the 
paper. The directions in which the comet 
and the earth are moving are indicated by 
arrows. ‘The positions of the two bodies on 
a number of dates are also given. ‘The peri- 
helion is reached on Jan. 25, 1884, when the 
comet is seventy million miles from the sun, 
and sixty-eight million miles from the earth. 
The nearest approach to the earth, about fifty- 
three million miles, is upon Jan. 8, 1884. 
The brightness, as far as depending upon 
the distance from the sun and from the earth, 
should reach a maximum about Jan. 11, a 
hundred and forty-five times as bright as when 
discovered by Brooks, and five times as bright 
as at the time of Bode’s observation, when, as 
already noted, the comet had a tail a degree 
in length. We might expect, then, that it 
aa--: would be visible to the naked eye 
from the middle of December to the 
middle of February, equalling, at its 
best, the brightness of a star of the third mag- 
nitude; but unusual and unexplained fluctua- 
tions in the brightness have been observed, 
which render these predictions a little untrust- 
worthy. In the first week in December the 
comet passed within about seven degrees of 
the bright star a Lyrae, and continued its 
motion rapidly towards the south and east. 
Since its discovery by Brooks, our visitor 
has behaved in a most peculiar manner as 
regards brightness. The theoretical change 
is given in Professor Boss’s article in Science, 
ii. 449. On the following page we find obser- 
vations made at Harvard college observatory 
on Sept. 21, 22, 23. The variability remarked — 
at Harvard is confirmed by observations made ~ 
at about the same time at Paris, Hamburg, and 
Dresden; so that we find a pretty well defined 
