=209. The Earth's Magnetic Field.=--Dr. William Gilbert's famous book, _De Magnete_, contains many helpful and suggestive ideas, none perhaps more important than his explanation of the behavior of the compass needle. He assumed that the earth is a magnet, with a _south-seeking_ pole near the geographical north pole, and with a _north-seeking_ pole near the geographical south pole. This idea has since been shown to be correct. The north magnetic (or south-seeking) pole was found in 1831, by Sir James Ross in Boothia Felix, Canada. Its approximate present location as determined by Captain Amundsen in 1905 is latitude 70° 5´ N. and longitude 96° 46´ W. The south magnetic pole is in latitude 72° S., longitude 155° 16´ E. The north magnetic pole is continually changing its position. At present it is moving slowly westward. [Illustration: FIG. 184.--Magnetic map of the earth for 1910. Isogonic lines ------ Isoclinic lines - - - -] =210. Direction of the Earth's Magnetic Field.=--Reference has been made to the fact that the compass does not always point exactly north. This indicates that the earth's magnetic field varies in its direction. Columbus discovered this fact upon his first voyage. The discovery alarmed the sailors since they feared they might come to a place where the compass would be unreliable. This variation is called _declination_. It is defined as the _angle between the direction of the needle and the geographical meridian_. Declination is due to the fact that the geographical and magnetic poles do not coincide. What is meant by a declination of 90°? Lines drawn upon a map so as to pass through places of the same declination are called _isogonic_ lines. The line passing through points where the needle points north, without declination, is the _agonic_ line. The agonic line is slowly moving westward. It now passes near Lansing, Michigan; Cincinnati, Ohio; and Charleston, S. Carolina. (See Fig. 184.) At all points in the United States and Canada east of the agonic line the declination is _west_, at points west of the agonic line the declination is _east_. =211. The Dipping Needle.=--Mount an unmagnetized steel needle on a _horizontal_ axis so as to be in neutral equilibrium, that is, so as to remain balanced in any position in which it is left. Upon being magnetized and placed so that it can swing in a north and south plane, the north-seeking pole will now be found to be depressed, the needle forming an angle of nearly 70° with the horizontal. (See Fig. 185.) The position assumed by the needle indicates that the earth's magnetic field instead of being horizontal in the United States _dips_ down at an angle of about 70°. Over the magnetic pole, the _dipping needle_ as it is called, is vertical. At the earth's equator it is nearly horizontal. _The angle between a horizontal plane and the earth's magnetic lines of force is called the inclination or dip._ [Illustration: FIG. 185.--A dipping needle.]