16 



TERRESTRIAL MAGNETlSiM. 



[Sect. II. 



measured distances from the centre of the suspended mag- 

 net, and perpendicular to it. The deflections thus pro- 

 duced, (/.^. the angular differences in the positions of rest 

 of the suspended magnet. P when influenced solely hy the 



Earth's magnetism, and 2° when in equilibrium between 



the Earth's magnetism and that of the deflecting maoiiet 

 at the distances employed,) furnish the ratio of the forces 

 exerted respectively by the Earth's force and that of the 

 magnet ; and as the p7-oduct of the same two forces is given 

 by the vibrations of the deflecting magnet when suspended 

 as in the experiments first described, the values of eitlier 

 force may be separately ascertained. The influence of the 

 magnetism of the magnet, and of its form and wei«.^lit, 

 being thus elinslnated, a m.easure is finally obtained of the 

 force of the Earth's magnetism, independent of the indi- 

 vidual properties of the m.agnet employed in the deter- 

 mination, 



4. Tlie numerical expression by which the measure of 

 the Earth's force thus obtained is denoted, depends on the 

 units of time, of 



siirements and calculation. In eonfurmitv with the In 

 structions published under the authority of the Royal 

 Society, a second of time, a foot of space, and a grain of 

 mass, are the units so employed ; and the horizontal com- 

 ponent of the Earth's magnetic force has been found, by 

 the observations hitherto made, to vary at diflferent points 

 of the Earth's surface from to about 8-4 of the scale 

 founded on the units which have been specified. 



5. T^Hierever the horizontal component of the force has 



pertained in absolute measure, there also, if the 



s 



pace, and of mass employed in the mea 



been 





magnetic direction be known, the " total force" in abso- 



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