196 On the Great Comet of 1843. 
West Point, May 23d, 1843. 
My dear Bache—The more immediate purpose of this letter, 
is to suggest to yourself and the Society, what has appeared to 
me a possible explanation of the very great discrepancies between ~ 
the observations and both the ephemerides computed from M. 
Arago’s and Mr. Walker’s elements. 
I suppose that the apparent orbit of the comet is different from 
the true; or that the path of the nucleus is not the same as that 
described by the centre of gravity of the entire mass. To illus- 
trate my meaning, suppose the comet to approach the sun ina 
parabolic or very elongated elliptical orbit, which will be, by the 
principles of physical astronomy, the path of the centre of grav- 
ity. As the comet approaches the perihelion, let it be greatly but 
gradually elongated in the direction of a line joining the nucleus 
and the sun, the tail being thrown off in a direction from this 
latter body, and suppose this to result from the repulsive action 
of the cometary particles upon each other, in consequence of the 
heating influence of the sun, in the same manner as the elastic 
force of vapor is increased by an elevation of temperature. The 
action being limited to the particles upon each other, the centre 
of gravity will be undisturbed, and continue to describe its reg- 
ular orbit from which each extremity of the elongation will re- 
cede on the line of the radius vector, though in unequal degrees, 
till it reaches a maximum, resulting from an equilibrium between 
the elastic force of the cometary medium and the weight of its 
elementary particles, or the force by which they are drawn to- 
wards the centre of the mass. 
The expansive action here supposed, would, in the nature of 
things, be gradual; and hence, before the nucleus, or the thing 
observed, could be totally resolved into a vapor like the tail, and 
thus disappear, the reverse action would begin, in consequence of 
the rapid retrocession of the comet from the sun. ‘The disturbed 
motion of the nucleus being for a part of the time from the true 
orbit, or that of the centre of gravity, towards the sun, the ob- 
servations, if made at this time, would give a constantly increas- 
ing eccentricity, or diminishing perihelion distance; and thus the 
perihelion itself might be brought apparently within the surface 
of the sun, while nota particle of the comet’s matter would 
touch that body. The observations, if made while the mass of 
the comet is contracting towards its centre of gravity, would give 
