\ of a Comet. 499 
they ihould be the mod: accurate that can be had, and from the 
difficulty of taking angles and pofitions of objects in motion.. 
Add to this a third caufe of error, namely, the obfcurity of 
very fmall telefcopic flars that will not permit the field of view 
fo well to be enlightened as we could wiffi, in order to fee the 
threads of the micrometer perfectly diftindh 
This will account for the apparent diftortions to be obferved 
in my figures of the Comet’s path. Some little irregularity 
therein may alfo proceed from different refractions, as they have 
not been taken into account, though the obfervations have 
been made at very different altitudes, where confequently 
the refradtions muff have been very different. But though 
this method may be liable to great inconveniences, the prin- 
cipal of which is, that many parts of the heavens are not fuf- 
ficiently ftored with fmall Bars to give us an opportunity to 
meafure from them, yet the advantages are not lefs remarkable. 
Thus we fee that it enabled me to diftinguiffi the quantity and 
diredfion of the motion of this Comet in a fmgleday (from the 
1 8th to the 19th of March) to a much greater degree of exadl- 
nefs than could have been done in fo ihort a time by a fedlor or 
tranfit infbrument; nay even an hour or two, we fee, were in- 
tervals long enough to ffiew that it was a moving body, and 
confequently, had its fize not pointed it out as a Comet, the 
change of place, though fo trifling as 2| feconds per hour, , 
would have been fufficient to occafion the difcovery, A gen- 
tleman very well known for his remarkable fuccefs in detedting; 
Comets * feems to be well aware of the difficulty to difcover a 
motion in a heavenly body by the common methods when it is 
fo very fmall ; for in a letter he favoured me with, fpeaking of, 
the Comet, he fays r Rien n’etoit plus difficile que de la 
“ reconnoitre et je ne puis pas concevoir comment vous aves pu 
Cl 
* Motif. MESSIER. 
reveiur ; 
