A chart of magnetic curves of equal variation / by Peter Barlow.Adam and Charles Black (Firm)Aikman, George, engraverBarlow, Peter, 1776-18621854]In lower right margin: Engd. by G. Aikman.Plate III-I from: General atlas of the world. Edinburgh : A. and C. Black, 1854.Prime meridian: Greenwich.World map showing magnetic curves.cartographic