THE USE OF THE MAGNETIC NEEDLE IN SURVEYING. 153 



Magnetic Declination. As an instrument for the determination of the 

 true bearings of lines, it is evident that the magnetic needle can be of 

 little value except as we are able to determine accurately its declination, 

 or the angle it makes with the true meridian. It is true that, when only 

 a comparison of directions is required, as in the survey of a field to deter- 

 mine its figure and area, it is of no consequence what the declination is, 

 provided it remains the same during the progress of the survey and for 

 all points where the needle is used ; but even then, to make the survey 

 useful in retracing the same lines at a future time, the declination should 

 be known and recorded. 



By observations upon the needle of a well constructed magnetometer, 

 the following facts relating to the declination will appear, some of which 

 will be indicated even by the ordinary compass needle. 



1. The declination is not the same in all places. 



2. For a given place it is subject to a secular change of unknown 

 period, but requiring at least several hundred years for its completion. 



3. It has a diurnal change, with a maximum and minimum for each 

 day. 



4. It has also an annual maximum and minimum, changing with the 

 seasons of the year. 



5. It is subject to irregular disturbances, being more or less affected by 

 every meteorological change. 



Discussing these in their order, we consider, 



i. The declination in different places. This is well shown by the chart 

 of the world (p. 8) upon which lines of equal declination, called isogonic 

 lines, are drawn. By reference to this chart it will be seen that, on this 

 continent, a line of no declination passes in a north-westerly and south- 

 easterly direction near Cleveland, O., and Raleigh, N. C. At all places 

 east of this line, the declination is westerly, that is, the north end of the 

 needle points to the west of north ; and west of the line the declination 

 is easterly. The map of New Hampshire and Vermont, herewith given, 

 shows the isogonic lines for these states, as delineated by the United 

 States Coast Survey. By observing the situation of a place with 

 reference to these lines, the declination for that place may be approxi- 

 mately determined ; but while they may be considered mainly correct for 

 this date (January, 1874), no surveyor should rely upon them for the 

 VOL. i. 21 



