THE MAGNETIC NEEDLE 
7 
dow glass may be cut down with a diamond and ground 
on a grindstone to fit its setting. It may then be set in 
place, with putty if possible, and the binding ring sprung 
into place over it. 
SECTION II 
THE MAGNETIC NEEDLE 
All compass surveying is based on the tendency of the 
magnetic needle to point north and south. The direction 
of the needle, however, is very far from being constant. 
Secular Change. There is a belt of country crossing 
the United States in a general north and south direction 
through the states of Michigan, Ohio, and South Carolina 
along which the needle at the present time points due north 
toward the earth’s pole. This belt is called the agonic 
line, or line of no variation. East of this line the needle¬ 
points westward of true north; west of this line it points 
to the eastward of it. The direction from any place toward 
the pole of the earth’s revolution is for that place the true 
meridian. The direction taken by the needle is the mag¬ 
netic meridian. The angle between the two is called the 
declination of the needle, west if the needle points west of 
true north, east if the needle points east of it. The declina¬ 
tion is greater the farther the agonic line is departed from, 
amounting to more than 20° in the maritime provinces and 
the Puget Sound country. The agonic line is not sta¬ 
tionary but is moving slowly westward, as it seems to have 
done constantly since the beginning of the last century. 
The declination of the needle, therefore, is changing from 
year to year and at a different rate in different parts of the 
country. 
These facts affect the work of the land surveyor impor¬ 
tantly, and sections on the bearing of lines and on ascer¬ 
taining the true meridian are given later on in this 
volume. 
Daily Change. The needle when free and undisturbed 
swings back and forth each day through an arc amounting 
commonly in the United States to about 10'. Early in the 
morning, from four to six o’clock according to the season, 
