THE USE OF THE MAGNETIC NEEDLE IN SURVEYING. 15I 



placed in the plane of the magnetic meridian. As we go towards the 

 south, the vertical component of this force diminishes, and the horizontal 

 component increases, as will be seen by the United States Coast Survey 

 chart (p. 6) showing lines of equal horizontal intensity, and, also, of 

 equal dip. Neither does the magnetic intensity remain the same for the 

 same place. Observations made at Washington, D. C, by the United 

 States Coast Survey, show that the total force at that place has heretofore 

 been slightly increasing, while at the present time it is nearly stationary, 

 or, perhaps, beginning to decrease. Like the other magnetic elements, 

 the intensity has its secular period of change, but the data are not at 

 present sufficient to determine that period ; and even if it were known, it 

 would be of no practical importance to the surveyor. 



Magnetic Dip. When the dipping-needle is placed in the plane of the 

 magnetic meridian, — that is, with its axis at right angles to this plane, — 

 the north end is drawn downwards, making, at Hanover, an angle with 

 the horizon of about 75° 30'. If, now, we carry this needle to the south, 

 we find the dip diminishing, until, near the equator, we reach a place where 

 it is zero. We may then trace a line, approximately east and west, upon 

 which there is no dip. North of this line the north end of the needle 

 will dip, and south of it, the south end. On each side of the line of no 

 dip, we may trace lines of equal dip called isoclinic lines. These lines 

 are shown, so far as they have been determined for the United States, 

 on the chart previously referred to (p. 6). Going northward, the dip 

 increases, till, at a magnetic pole, the needle takes a vertical position. 



The magnetic dip, like the intensity, is slowly changing, as continued 

 observations upon the dipping-needle show. Previous to 1854, it was 

 increasing in the United States, and since that date it has diminished 

 about 30'. 



The use of the magnetic needle in surveying does not require special 

 attention to the dip. It is only necessary to place upon one end of the 

 needle a suitable counterpoise to keep it in a horizontal position, since, 

 when balanced before being magnetized, it will always require such a 

 counterpoise after it is magnetized, unless used upon the line of no dip ; 

 and, when balanced for one latitude, it will need readjusting if taken to a 

 different latitude. When the needle is once properly balanced for any 

 place, the surveyor need give no further attention to the dip. 



