1 7 8 SECTION PHYSICS. 



intended, therefore, to record changes in the direction in which the 

 earth's magnetism is acting. The second instrument is called a bifilar 

 magnetometer : it is a magnet suspended by two parallel or nearly 

 parallel threads, and the upper attachment of those threads is twisted, 

 so as to bring the magnet round exactly into the position of magnetic 

 east and west, that is, at right angles to the magnetic meridian. In 

 that position the tendency of the suspended weight to untwist is 

 balanced by the pull of the earth ; and supposing its tendency were 

 to untwist towards the south, the magnetism of the earth tends to pull 

 the marked end of the magnet towards the north, and it is so 

 arranged that the torsion force is exactly balanced by the magnetic 

 force. Now if any increase of magnetic force takes place, it would 

 tend to overcome the torsion force, and to pull the marked end of 

 the magnet further towards the north ; but if the magnetic force of 

 the earth diminishes, the torsion force will preponderate and the 

 magnet will untwist a little, and it will move slightly in the contrary 

 direction. The third instrument is what is called the balanced mag- 

 netometer, which I hold in my hand. For that purpose a magnet is 

 balanced on a knife-edge resting on an agate plane, and placed at right 

 angles to the magnetic meridian, so that while the weight of the magnet 

 tends to elevate the marked end, the vertical force of the earth draws 

 the marked end downwards, and that force will deflect a dipping-needle 

 about 70 to the horizon. The vertical force of the earth tends to 

 draw the marked end down, and if it diminishes, the weight would 

 cause that end to move in the other direction and rise a little upwards ; 

 consequently the movements of this magnet in a vertical plane indicate 

 the changes which take place in the earth's vertical force. 



For many years observations were made with these instruments 

 solely by means of the eye with the aid of a telescope ; and they 

 were made by attaching a small mirror to the magnet, which was viewed 

 by a fixed telescope with cross-wires in it, and by the telescope a fixed 

 scale placed at a certain distance was seen by reflection from this mirror ; 

 and as the magnet moved, and, consequently, the mirror moved with it, 

 it is clear that if the cross-wire in the telescope were adjusted so as to 

 fall on the scale, as the mirror moved, the cross-wire would appear to 

 rise or fall upon the scale, and by that means the position of the 

 magnet at the time was recorded simply by eye observation. But in 



