from The Century Dictionary and Cyclopedia
- n. A chart on which lines, called isogonic lines, are drawn passing through places having the same magnetic variation. See cut under isogonic.
Sorry, no etymologies found.
In this voyage he determined the longitude of several places; and, after his return, constructed his variation-chart, and proposed a method of observing the longitude at sea, by means of the appulses and occultations of the fixed stars.