166 NAVIGATIONAL COMPASSES
line is an irregular curve which in the western hemisphere extends from longitude about 96° W., at latitude 70° N., to longitude about 28° W., at latitude 70° S.; and in the eastern hemisphere extends from longitude about 28° E., at latitude 70° N., to longitude about 138° E., at latitude 70° S. A compass needle on an agonic line places itself in the geographic meridian, that is, points true north and south. At various places on the earth, a compass needle that is uninfluenced by any magnetic field except that of the earth will show declinations as great as 180 degrees. In the state of Maine the compass points about 25 degrees west of the geographic meridian, and in the state of Washington it points to the east an equal amount. The compass declination at any point on the earth changes with time. At New York harbor the declination now is about 11 degrees and is increasing at the rate of 6 minutes per year. The magnetic declinations are known for all parts of civilized lands and navigated areas. They are not known, however, for large areas within the arctic and antarctic zones.
The directive tendency of a magnetic compass depends upon the magnitude of the horizontal component of the earth's magnetic field. At no place within either the arctic or the antarctic zone is the horizontal component greater than about one-half the value in New York. Within considerable areas it is nearly zero.
100. The Deviations of a Magnetic Compass
or. an Iron Ship. - While a ship is being built, the magnetic field of the earth causes the iron structure to become a big magnet with the north-seeking pole toward the north. The various hammering operations facilitate the magnetic induction. If the ship is heading north while being built, there will be developed a north-seeking pole at the lower part of the bow and a south-seeking pole at the upper part of the stern. The steel parts of the ship will retain a part of this magnetism after the ship is launched and is pointing in any direction. A ship that was built with the keel north and south will give no deviations due to this subpermanent magnetism when pointing either north or south. As the ship is rotated 360 degrees, the compass will deviate to the east in one semicircle and westerly in the other (Fig. 128). Deflections of the compass from the magnetic meridian due to this cause are called semicircular deviations. In many cases, semicircular deviations amount to as much as 20 degrees. The semicircular deviation of a compass changes with change of geographical position. If the bow of the ship has been toward the east when building, a north-seeking pole would be