Sect. II.] 



TERRESTRIAL MAOXETISM, 



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Appendix No. 1. A tolerably practised obs.. ,er will 

 ooraplete the procc^ss by which a measure of the absolute 



horizontal force is cl.tained in about two hours, includin 

 the time required for setting up and adjusting tlie instru- 

 ment. It is desirable that there should be at least five or 

 six repetitions at places which are to serve as base stations ; 

 nd also, as a spare rnagnet is always supplied, that both 

 agnets should be employed. 



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There are certaii 



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otlrrup in which it rests, the change which the magnetic 

 moment of the magnet undergoes from an alteration of 



one degree of temperature, and the coefficient in the cor- 

 rection required for an increase of force which the magnet, 

 in certain positions in which it may be used, r^ay receive 

 bv hiduction from the earth,] which have to be determhied 

 for each magnet once for all, and require for their deter- 

 mination apparatus which is not afterwards needed : these 

 constants have hitherto been usually determined at IVool- 

 wich before the instrument is put into the hands of the 

 officer who is to use it elsewhere. 



Relative Measurements of the Magn^Llc Force. 



7. These are the observations which are made at sea, to 

 determine the ratio of the total force in the geographical 

 position of the ship at the time when the observation is 

 made, to ^^ value at some base station where the instru- 

 nient has been landed and used in precisely sim.ilar 

 observations to those made on board ship. The instru- 

 ment is the well-known apparatus devised by Mr, Fox, 

 which has contributed more to a knowledge of the geo- 



raphical distribution of terrestrial magnetism than any 



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