THE USE OF THE MAGNETIC NEEDLE IN SURVEYING. 1 49 



directions, will take a position in the magnetic meridian which (if the 

 observation be taken at Hanover) will deviate from the true meridian 

 about 11°, the north end of the magnet turning to the west of north. 

 Moreover, also, it will incUne to the horizon, the north pole dipping down- 

 ward at an angle of about 75° 30'. This is called the inclination or dip 

 of the needle. It becomes necessary, therefore, in studying the phenom- 

 ena of terrestrial magnetism, to make use of two instruments, — one for 

 observations upon the declination, and the other upon the dip of the 

 needle. In the former, the needle hangs horizontally in a stirrup sus- 

 pended by a fibre of untwisted silk, which leaves it free to turn in a 

 horizontal plane with the least possible resistance ; while the latter, called 

 the dipping needle, is balanced upon a horizontal axis, and is free to turn 

 only in a vertical plane, and when in use must have its axis perpendicular 

 to the plane of the magnetic meridian. Besides the declination and dip, 

 we may also consider the intensity of terrestrial magnetism, by which is 

 meant the amount of that force which restores the needle, when dis- 

 turbed, to its normal direction. This element is of so little practical 

 importance in the ordinary use of the needle, that it may be passed 

 briefly. 



Intensity of Terrestrial Magnetism. If a magnetic needle, — suspended, 

 as mentioned above, by a fibre of silk, — be drawn out of the magnetic 

 meridian by bringing near it another magnet, and then allowed to return 

 by removing the second magnet to a distance, it will oscillate for a time 

 before the resistance of the air and of the suspending fibre will bring it 

 to rest. If the weight and dimensions of the needle are accurately known, 

 and the number of oscillations it makes in a given time be observed, it is 

 easy to compute the intensity of the force which actuates it, the more 

 rapid oscillation indicating the greater force. This, however, will not 

 represent the total force of the earth's magnetism, but only that part of it 

 which tends to bring the needle into the plane of the magnetic meridian, 

 and which is called the horizontal intensity, or the horizontal component 

 of the magnetic force. The vertical component tends to draw the north 

 end of the needle downward (in the Northern hemisphere), causing the dip. 

 The actual direction of the force of terrestrial magnetism at any place is 

 the same as that of a magnetic needle suspended from its centre of 

 gravity, and free to move in all directions, or of the dipping-needle when 

 VOL. I. 20 



