We present both a description of a new magnetic data set covering predominantly the 18th and 19th centuries and the results derived from it for the small window 1820--1850, from which the bulk of the data originate. The data set comprises measurements of declination taken overwhelmingly on French naval and hydrographic vessels. A list of the vessels is given for one of the data sets. When augmented by extant inclination measurements, the data are capable of resolving the magnetic field at the core-mantle boundary to a high degree of fidelity and thus are a valuable addition to the data set of historical geomagnetic measurements.