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X. A new Method of deducing a first Approximation to the Orbit 
of a Comet from three Geocentric Observations, By James Ivory, 
A. M. Cojnmunicated by Henry BroiJgham, Esq. F. R. S. 
Read February 17, 1814,. 
Comets are distinguished from the planets not only by the 
peculiarities that immediately strike the eye, but likewise by 
the circumstances attending their motion in the heavens. All 
the planets move round the sun in orbits nearly circular; they 
new'r deviate far from the ecliptic on either side ; and they 
move in a manner not extremely irregular, and in one direc- 
tion, according to the order of the signs in the zodiac. Comets, 
on the contrary, when they hrst come into view, assume gra- 
dually greater degrees of brightness, which they again lose 
by like gradations, and then disappear ; thus seeming to visit 
the neighbourhood of the sun for a short time only, after which 
they retire into the immensity of space: they are seen in all 
quarters of the heavens : and their motion is exceedingly va- 
rious and irregular; confined to no direction; sometimes 
greatly curved, and often hardly distinguishable from a rec- 
tilineal course. If, to phenomena so dissimilar, we add the 
prejudice which almost universally prevailed, that comets 
have only a temporary existence, and are produced by occa- 
sional causes, we shall not perlia])s have much reason to be 
sui'prised that the true account of those bodies, which repre- 
sents them as forming a part of the same system with the 
planets, eluded the sagacity of Kepler, to whom we are 
MDCCCXIV. R 
