of a Comet . 499 
they mould be the moft 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 o£ 
very fmall telefcopic ftars that will not permit the field of view 
fo well to be enlightened as we could wifh, in order to fee the 
threads of the micrometer perfectly diftinCh 
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 refraftions 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, fuff 
ficiently ftored with fmall ftars to give us an opportunity to 
meafure from them, yet the advantages are not lefs remarkable. 
Thus we fee that it enabled me to diftinguifh the quantity and 
direction of the motion of this Comet in a fingle day (from the 
1 8th to the 19th of March) to a much greater degree of exact- 
nefs than could have been done in fo fhort a time by a feCtor or 
tranfit inftrument ; nay even an hour or two, we fee, were in- 
tervals long enough to fhew 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 feeonds per hour, 
would have been fufficient to occafion the difcovery. A gen- 
tleman very well known for his remarkable fuccefs in detecting 
Comets * feems to be well aware of the difficulty to difeover 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 : 44 Rien ii’etoit plus difficile que de la 
45 reconnoitre et je ne puis pas concevoir comment vous aves pu 
* .Monf, MESSIER, 
44 revenir 
