Historical notes on the deviation of the compass

Abstract

Few historians of the magnetic compass carry their story beyond the period of the birth of ocean navigation. To those interested in this fascinating instrument there are, however, many interesting lines of enquiry to be traced in the centuries which followed. These notes are the result of an attempt to investigate the beginnings of just one branch of progress.During the Sixteenth, Seventeenth, and Eighteenth Centuries, when there were many who tried to solve the problem of finding the longitude at sea, one of the favorite methods proposed was the use of the variation of the compass. Of those who proposed this method one of the last was Ralph Walker, a resident of Jamaica, who, in the year 1793, traveled to England to lay his proposals before the Board of Longitude. It is not intended here to discuss his variation theory but to point out that, in addition to producing this theory, he designed a compass to enable the variation to be observed with greater accuracy at sea. This compass was sent for trial in H. M. Ships Invincible, Glory, and Lynx and it was this trial in the Glory which introduced her master, Murdo Downie, to fame among those who have written on the magnetic compass. In his report on Walker's compass Downie wrote [see 1 of “Authorities quoted” at end of paper]:“It appears, that the variation observed at one view by Walker's compass, and that observed by the ship's compass by the bearing and altitudes, were generally very near the same. But it is evident, that the variations given by both compasses at different times and situations disagree very much: Whether any part of this disagreement may be owing to the time of the day the variations were taken, I cannot take on me to determine; but I am pretty well convinced, that the quantity and vicinity of iron in most ships has an effect in attracting the needle; for it is found by experience, that the needle will not always point in the same direction when placed in different parts of the ship: Also it is rarely found, that two ships steeling the same course by their respective compasses will go exactly parallel to each other; yet these compasses, when compared on board the same ship, will agree exactly.”