206 On the Great Comet of 1843. 
forces of the sun and comet. If the one is determined by meas- 
ure, the other can be computed from it. Olbers made measures 
of the shape of the visible section of this hyperboloid. 
Brandes* gave this theory a thorough discussion, and finds 
analytically that the opinion of Olbers is true, that the envelope if 
so caused must be an hyperboloid, and then from the observed 
dimensions of a section of this hyperboloid as seen from the earth 
computes the ratio of the two repelling forces for that comet. 
The theory of the formation of comets’ tails seems to have 
made but little advances from 1812 till the return of Halley’s 
comet in 1835, when the astronomers of Europe, with a full 
knowledge of Olbers’s theory, and with powerful instruments, 
observed the tail and nebulosity of that comet, with reference to 
this theory.t Struve remarked that the densest point of the ne- 
bulosity was eccentric in that nebulosity, conformably to the ob- 
servation of Messrs. Herrick and Bradley for the recent comet. But 
the most indefatigable observer was Bessel,{ with the Konigsberg 
heliometer. He detected a pendulous or vibratory motion of that 
portion of the nebulous matter, which was expelled from the hemi- 
sphere of the comet next the sun, resembling that of a magnet 
round the magnetic pole. He finds that with the addition of this 
pendulous motion of this streaming matter, or in other words of 
the apex of the hyperboloidal envelope of the comet in the plane 
of the orbit, and for fifty degrees or more on each side of the 
antipode of the comet’s tangential direction, all the observed 
phenomena of the tail of Halley’s comet may be explained. 
Bessel then proceeds to compute the repulsive force of the sun 
and comet on these expelled particles from the observed shape of 
the tail. He also computes for any point of the tail how long 
the expelled particle had taken since the date of its expulsion to 
arrive at the point observed. 
All these phenomena Bessel explains upon the supposition that 
there is no molecular attraction or repulsion between the particles 
thus expelled; but that their heliocentric motion is due to the 
tangential direction in which they are expelled from the comet, 
and to the two forces of repulsion of the sun and comet, and the 
* Monatliche Correspondenz, Vol. xxv1, p. 533. 
+ Schumacher’s Astronomische Nachrichten, Vol. x111, p. 303. 
$ Ibid., Vol. x11, p. 177. Also Schumacher’s Jahrbuch for 1837, p. 142. Con. 
des Tems, 1839. 
