This unusual and scarce pair of globe gore sheets was issued in the second edition of John Churchman's The Magnetic Atlas, or Variation Charts of the Whole Terraqueous Globe, published in London (the first edition of 1790 has a slightly different title). Early in his career, Churchman was a surveyor in the American colonies, including stints in both Delaware and Pennsylvania. In the 1780s, Churchman was captivated with the issue of determining longitude at sea. While John Harrison and others had found a solution for this issue with the chronometer, Churchman took a completely different and more cost effective approach to this question. Churchman's premise was that the northern and southern magnetic poles were fixed and that magnetic variation along any line joining these two points was constant. Therefore, a mariner only needed to know his latitude and magnetic variation to calculate exact longitude. Unfortunately, Churchman's theory was flawed as both poles and variation are constantly changing.

Each plate contains 12 globe gores which, if assembled, would create a globe measuring 48" in circumference and about 7.6" in diameter. Both sheets contain a magnetic pole, magnetic orbit, and various meridians linking the two poles together. Magnetic north is located in northern Baffin Bay, but its precise location wouldn't be discovered until 1831 by James Ross (magnetic south wouldn't be determined until 1909). Geographically, the continents are presented fairly accurately in outline form with little additional detail.

A fascinating pair of maps that rarely come on the market.

References: Sabin #13026; Wheat & Brun #6; cf. Pritchard & Taliaferro pp. 232-35.

Condition: B

Issued folding on watermarked paper with numerous archivally repaired fold separations along the horizontal folds. There is some small loss mostly along the upper horizontal fold of the northern sheet and minute loss at the fold junctions of both sheets. Each sheet has minor foxing and light toning along a few folds.