198 On the Great Comet of 1843. 
March 20.5. R.A. —0”.6 Dec. +0”.7 
"30:6 R, A. —0 0 Dec. —1.0 
April 9.5 R. A. —0 .6 Dec. +0 .3 
These normal places were obtained from a comparison of all 
our observations with the best ephemeris we could obtain, which 
was computed from our elements at our request, by Mr. John 
Downes, the editor of the United States Almanac, and obtained 
from the average corrections concurring together near the 20th 
and 30th of March, and 9th of April, for Greenwich mean mid- 
night. These are far more correct than the result of any single 
measure. We give them for the use of astronomers freed from 
refraction, parallax, and aberration. - 
March 204.5, R. A. 46° 4/38”.4 Dec.S.9° 9/ 45.5 
March 304.5, 59. bk 1, 2 6 36 32 .5 
April 945, 68 56 41 6 4 AB 36 .7. 
_ Let us now consider the period belonging to the mean motion. 
It is obvious that if we adopt the explanation of Messrs. Alexan- 
der and Bartlett, the mean motion and consequent period of the 
centre of the nebulosity observed, and of the real centre of gra- 
vity, must be the same. This is a necessary condition, since 
they both arrive at the perihelion point at the same instant of 
time. Now the earth’s sidereal. motion in a mean solar day is 
3548”.18761. The mean motion of the apparent centre of the 
nebulosity by our elements is 159”.58936. This gives a period 
for the apparent centre of the nebulosity, and consequently for 
the actual centre of gravity, of 22.2339 years. 
This is the keystone of the arch; it is the last argument that 
was wanting to complete the conclusion. Analogy had already 
raised a violent presumption of a period of 21% years.. ‘The same, 
or nearly the same period, has here been derived by a process en- 
tirely independent of these analogies, and entirely free from any 
hypothesis respecting the period or nature of the conic section 
that forms the orbit. The coincidence is wonderful, and shows 
not only the strong probability of the period, and of the identity 
of the three comets, but also the extreme precision of the notmal 
places, derived from our measures with the filar-micrometer ; for 
an error in any of these places of 10” would have led to a greater 
discrepancy. ss 
We have presented the argument a posteriori, from the nature 
of the orbit observed in March and April last. We have found a 
