( X4 ) 
their refpe&ive Points in the Axis as ordinates, the ex- 
tremities of chofe Lines fhali touch the Curve of a Pa- 
rabola ; as may be feen in the Figure : Where a, b, c, e, 
being fuppofed Points obferved, the Lines aB, bC, c A, 
eF, are refpeftively proportional to the times of each 
Obfervation before or after the Tropical Moment in 
Cancer. 
This premifed, we fliall be able to bring the Problem 
of finding the true time of the Tropick by three Ob- 
fervations, to this Geometrical one : having three Points 
in a Parabola A,B,C, or A,F,C given, together with 
the diredrion of the Axis, to find the Diftance of thofe 
Points from the Axis. Of this there are two cafes, the 
one when the time of the ftcond Obfervation B is pre- 
cifely in the middle time between A and C : In this cafe 
putting t for the whole time between A and C, we lhall 
have Ac the interval of the remcteft Obfervation A 
from the Tropick by the following Analogy. 
As X a c— be to 1 a c — i b c : : So is i t or AE : to A c 
the time of the remoteft Obfervation A from the Tro- 
pick 
But the other cafe when the middle Obfervation is 
ml exadly in the middle between the other two times, 
as at F, is fomething more operofe, and the whole time 
from A to C being put = tjand from A to F =s, ce = c, 
1 1 c b s s 
and b e = Lthe Theorem will {land thus .=Ac 
xtc — lbs 
the time fought. 
To illuflrate this Method of Calculation it may per- 
haps be requifite to give an Example or two for tlic fake 
of thofe Aftronomers that are lefs inftrufted in the Geo- 
metrical part of their Art. 
Anno \soo Bernard Walt her in the Month of June at 
MuremMrg obferved the Chord of the diftance af the 
San from th^ Zenith by a large Parallaftick Inflrument 
©£ Ptokmj^ as follows : 
