On the observations of Comets. 95 
the comet of 1695, is known only by observations made at 
sea by a French missionary: several others also are known 
e 
any observatory. It is therefore, highly desirable for the ad- 
vancement of astronomy, that all lovers of science should 
make all possible observations whenever a comet appears. 
The want of proper instruments, is indeed a great im- . 
pediment; but even without a telescope, the positions of 
a comet might be ascertained with sufficient accuracy by 
measuring with a circle of reflection, or a sextant, its 
distances from two other heavenly bodies aes positions 
are exactly known. pte method of which He pew s and 
tronomical instruments could be used only with “liffealt y. 
lic, the following essay, on the manner of ascertaining the 
positions of comets from the distances observed. 
Fig. 1. 
P Let p (fig. 1,) be the pole of the earth, ¢ 
the place of a comet, and a and 6 two -aig 
ed stars. Pe will be the comet’s polar di 
tance, and the angle cpa the difference og 
tween its right ascension and the right as- 
cension of the stara. Measure the distan- 
@ cesca and cb from the comet to each star, 
ana mark the time. The distances may be taken simultane- 
ously when there are two observers ; but when there is only 
one, the distances from one star must be reduced to the time 
at which the distances from the other star were taken. A 
circle of reflection would be the best instrument for these 
observations; but even with a sextant, the distances may be 
pads oa great accuracy, by taking the mean between several. 
The distances will be affected by the 
z relent: ee may be reduced to true 
, Letzbe the zenith, e and athe true pla- - 
S 
@ apparent places. Making c cz=N, az= 
c N’, and c'a’=D we hav 
