20 ON MAGNETISM. 



verified, by their power of deflecting a compass-needle.) 

 The apparatus is represented in Figure 8. Suspend the 



Fig. 8. 



axis delicately; load it with each of the needles in 

 succession, one at a time ; observe the time of vibration 

 as produced by terrestrial magnetism ; and, if they 

 differ slightly, take the mean. Then place all the 

 needles on the axis, and it will be found that the time 

 of vibration is the same as that mean. This shews 

 that the terrestrial magnetic statical force on the 

 assemblage of needles bears the same relation to the 

 moment of inertia of the assemblage as that which 

 existed for a single one : and therefore that the ter- 

 restrial magnetic statical force is proportional to. the 

 number of similar magnets on which it acts. 



(6) The direction in which terrestrial magnetism 

 acts being known, and a line being drawn through 

 the center of the axis at right angles to that direction, 

 place in that line a magnet (either similar to one of 

 the needles, or of any other form and magnitude) which 



