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X 



ERRE::..RIAL MAGNETISM. 



[Sect, II. 



Other recent invexition. Trie following brief description 



'\ general idea of the apparatus and 



mav serve to give 



of the mode of obtaining results with it, more full 

 directions for its use heing given in Appendix No. 2. It 

 con?istR of a dipping-needle and gi-aduated circle, differ- 

 ing little from the accustomed form of an Inclinometer 

 except that the needle is supported by the ends of the 

 axle, which terminate in cylinders of small diameter 

 working in jewelled holes. A small grooved wheel is 



carried on the axle, and receives a thread of 



unspun silk, 



farnislied at each extremity with hooks to which small 

 weights may be attached, for the purpose of deflecting- 

 the needle from its position of rest in the magnetic direc- 

 tion, and causing it to take up a new position, in whicli it 

 is in equihbrium between the opposing forces of the 

 Earth's magnetism and of the deflecting weight. The 



weight being constant, and the magnetism of the needle 

 assumed to be so, the intensity of the Earth's magnetic 

 fsjrce in different localities is inversely as the sines of the 



For'gTeater accuracy, several con- 



angles of deflection 



stant weights are employed on each occasion ; and each 



weight is successively attached to each of the two hc^Li, a 

 mean being taken of the deflections on either side of the 



at sea is 



position of rest. 



The apparatus when 



placed on a giml):ul toble, by which the motion of the 

 vessel is greatly counteracted : and when the weather does 

 not permit the manipulation of the weigLL, deflecting 

 magnets are 





bstituted, the operation of whicb may be 

 understood from the detailed instrur^'ons in Appendix 



"^ J. X 



No. 2, A\'ith the gimball table as recently constructed, 

 it is found that but very few days occur in which the 



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