CHANGE IN DIRECTION OF EARTH’S MAGNETIC FIELD. 273 
and the vertical plane passing through the magnet, or through 
the line of force, is the magnetic declination, or the variation 
of the compass; the angle between the horizon and the direc¬ 
tion of the needle, measured in the vertical plane passing 
through it, is the dip or inclination; and the force with which 
the needle is held in the direction of the lines of force is called 
the magnetic intensity. The declination and inclination, or 
the directional elements, which alone are concerned in a dis¬ 
cussion of the direction of the magnetic field, have generally 
been treated separately in investigating the secular change 
of the magnetic needle. From 1634, when the fact of the 
secular variation of the declination was established, and from 
1576, when the inclination or dip was discovered, reliable ob¬ 
servations of these respective elements are recorded for the 
great populous centers of Europe, and soon observations of 
the declination or variation of the compass, a knowledge of 
which is necessary to mariners in the navigation of their 
ships, had been made by navigators in most of the known 
parts of the world. Although the older observations, having 
been made without the means of precise measurement, are 
subject to a probable error of as much as 1°, they can be 
accepted as serviceable in the discussion of long series, and 
serve to reveal satisfactorily the secular change of the decli¬ 
nation. Through the results of the observations of the nav¬ 
igators of successive periods, series of observations of the 
declination extending over two or three centuries are avail¬ 
able for most of the important maritime stations of the world. 
An examination of the curves resulting from platting the 
observed and computed values of the declination at a few 
stations, where the series extend over the greatest duration 
and are the most complete, will show upon what evidence 
rests the widespread belief that the secular variation of the 
magnetic declination is a periodic phenomenon. 
There are also available for discussion series of observa¬ 
tions of the dip or magnetic inclination ranging from one 
hundred to three hundred years in duration., but the stations 
are not so numerous nor the observations so complete as in 
