An explanation of the magnetic atlas, or variation chart, hereunto annexed : projected on a plan entirely new, by which the magnetic variation on any part of the globe may be precisely determined, for any time, past, present, or future: and the variation and latitude being accurately known, the longitude is of consequence truly determined. By John Churchman, late land surveyor for the district of the counties of Delaware and Chester, and for part of Lancaster and Berks, Pennsylvania.