The Collaborative International Dictionary
magnetic dip \magnetic dip\ n. The angle that a magnetic needle makes with the plane of the horizon.
Syn: dip, angle of dip, magnetic inclination, inclination.
Wiktionary
n. The angle between magnetic north and the horizontal (as shown by a compass held vertically).
WordNet
n. (physics) the angle that a magnetic needle makes with the plane of the horizon [syn: dip, angle of dip, magnetic inclination, inclination]
Wikipedia
Magnetic dip, dip angle, or magnetic inclination is the angle made with the horizontal by the Earth's magnetic field lines. This angle varies at different points on the Earth's surface. Positive values of inclination indicate that the magnetic field of the Earth is pointing downward, into the Earth, at the point of measurement, and negative values indicate that it is pointing upward. The dip angle is in principle the angle made by the needle of a vertically held compass, though in practice ordinary compass needles may be weighted against dip or may be unable to move freely in the correct plane. The value can be measured more reliably with a special instrument typically known as a dip circle.
Dip angle was discovered by the engineer Georg Hartmann in 1544. A method of measuring it with a dip circle was described by Robert Norman in England in 1581.
Usage examples of "magnetic dip".
That would acccount for magnetic dip because the compass would point straight at the mountain through the bulge of the spherical Earth.
When she had first come on watch she had obtained observations--a position line by magnetic dip, crossed by a line of soundings as they passed over the Clarendon Deep.