Professor Gautier on the Comet of July 18 25. 307 
the orbit of the comet itself. For this purpose, a line SE 7 is 
supposed to be drawn upon the orbit, making with the line of 
the nodes NN' an angle equal to that comprehended upon the 
ecliptic between this latter line and the line SE ,* and the angle 
E'SP reckoned in the order of the signs, from 0 to 360° pro- 
ceeding fromE', is what is called the longitude of the perihelion. 
I have found this angle thus reckoned 318° 34?', which gives 
41° 26' for the acute angle PSE', which is its complement to 
360°. 
The direction of the comet’s motion being from N toward O, 
we find that this direction projected upon the ecliptic, and, seen 
from the sun, is contrary to that of the earth’s motion upon its 
orbit, or to the order of the signs, which is from E towards o. 
This is what is expressed by saying that the heliocentric motion 
of the comet is retrograde *. 
After having presented the approximative elements of the or- 
bit of the comet, there remains for me to develope the conse- 
quences deducible from them, following it in its progress from 
the first moment of its appearance, and pointing out its succes- 
sive distances from the sun and the earth, as well as the geo- 
centric positions which it must have assumed since its disap- 
pearance. 
At the moment of its discovery, which was made on the 15th 
July at Lucques by M. Pons -f-, and, on the 19th at Prague, 
by M. de Biela, the comet was at D, at a distance from the sun 
S of about twice and two-fifth times that of the earth, and at a 
* It is known that this alternative of direction is peculiar to this kind of stars, 
while all the planets and satellites whose motion is well ascertained move in the 
right direction. Of the 129 comets whose orbits are now determined, there are 
68 in which the motion is direct, and 61 in which it is retrograde. 
*|- M. Carlini seems disposed to think (Corr. Astr. t. 13. p. 291.) that it is the 
comet of Encke, and not the great comet which M. Pons discovered on the 15th 
July. My elements, however, give me for that day the same declination as that 
resulting from M. Pons’s estimate, and a right ascension, which differs only a 
few minutes of a degree from his. However this may be, the two comets must 
have been, at this period, in very near geocentric positions, and it would be singu- 
lar if no person had observed both of them at once at this moment. The comet of 
Encke could only have been then at a distance from the earth, nearly equal to 
three-fifths of that of the other comet. 
