[ 349 ] 
radius. For expedition thus ; draw two lines, make- 
ing an angle of 17® ^ 6 ' : on one of them fet off the 
perpendicular from the comet’s projedled place, and 
raife a perpendicular to the other ; or, which is the 
fame, from the comet’s real place in the parabola ; 
and let fall a perpendicular, that is the tangent of 
the geocentric latitude. 
One obfervation of a known comet will, on fuch 
a fcheme, determine in fome meafure its whole 
courfe ; for, from the earth’s place, draw the ob- 
ferved longitude of the comet, where that cuts the 
projed;ion of the parabola is the comet’s place j to 
which if the obferved latitude agrees, it confirms it :• 
Then the other data being already known, and one 
place given, its whole courfe may be traced. Such 
a fcheme may be alfo of ufe to find the periods of 
comets, where the defer! ption- of one is not good 
enough to find its orbit by ; for if an old comet was 
feen in Auguft, in or in $, with fouth latitude, 
or very bright in January, it cannot be the comet of 
1682; but if in November in b, near the ecliptic, 
it may. It then remains to fee, whether the reft of 
the defeription will agree with the courfe it would^ 
in that cafe take : if it does, then, as the account is 
more or lefs perfed:, there is a greater of lefs proba-- 
bility of its being the fame. (See Plate XJ.) 
A‘ Table 
