AGONIC LINES (from ά privative, and γωνία, an angle), the imaginary lines on the earth’s surface where the magnetic needle indicates no declination or deviation from the terrestrial meridian—that is, points to the true north and south. There are two great primary agonic lines, varying from time to time, the courses of which for the epochs 1787 (from Hansteen’s Magnetismus der Erde) and 1840 (by General Sir E. Sabine) are figured in Keith Johnston’s Physical Atlas.