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In the 1945 annual book from the Handels- og Sjøfartsmuseet på Kronborg, now the Maritime Museum of Denmark, in Elsinore, there is a paper by Carl V. Sølver (1882-1966) titled "About the determination of the geographical longitude by the variation of the compass". The paper deals with the history of longitude by variation. It is written in Danish. One section says:

"Then, apart from the traverse calculation, you only had the ancient method of magnetic variation, and although there is no evidence in written documentation so far known, we know after oral tradition that it has been regularly used. Thus told the late Professor Schwartz, Director of the Copenhagen School of Navigation, that in his youth — about 1850 — the masters of the Danish schooners that sailed with salt from St.Yves in Spain to Newfoundland and from there to Denmark with stockfish, understood only calculation of latitude, but knew nothing about longitude calculation by astronomical observations; there was no talk about chronometers for such people at all. They sailed on a north-west course in the Atlantic Ocean until reaching Newfoundland latitude, after which they followed this parallel of latitude westwards until the land came into view. An approximate longitude was calculated according to the traverse, i.e. by noting up the sailed courses and the speed measured with a hand log. Every time there was a good opportunity to do so, e.g. when the Polar Star was visible, observations of variation were taken to determine the longitude. Professor Schwartz further reported that he himself on a journey from Copenhagen to Godhavn with the Royal Greenland Merchant ship "Thorvaldsen", Capt. Jensen, investigated the applicability of the method and found that for the determination of the longitude to within a few degrees one may well use it, insofar as one has a chart of variation brought up to date and a good instrument for taking bearings."

These schooners were built of wood so the deviation was likely insignificant. The paper is enclosed if someone likes to translate it. Using an on-line tool you may expect some strange wording, e.g. "cutlery bill" means traverse calculation, in Danish "bestikregning", where transverse=bestik=cutlery and calculation=regning=bill.

This method was used on small Danish schooners well into the first half of the 20th century according to hearsay.

Lars