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, 



