So of the 44 comet discoveries in the time between 1758 and 1801, when Messier was successfully observing, the two astronomers were involved in 27.
Gary W. Kronk's resources include the stories of historic observations, so among others, those of Charles Messier.
More data on Messier's comet discoveries can be found in Don Machholz's book, The Messier Marathon Observer's Guide, page 2.
Last Modification: 7 Feb 1998, 18:10 MET