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Sect. Ii.j 



TERRESTRIAL MAGNETISM. 



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lute measure is determined; since it consists of the hori- 

 zontal component multiplied by the secant of tlie angle 

 which the macrnetic direction makes with the horizon. As 



ships are supplied with instruments by which this angle, 

 called the dip or inclination of the needle, is measured, 

 the observations on land, when the ship is in harbour, give 

 determinations of the total force, which serve as base deter' 

 minations, to which are referred the relative results ob 

 tained at sea in the passage from one station of well- 

 assured ahsolute determination to another; — a practice 



corresponding to that which prevails in determinations of 



longitude, where stations of w^ell-assured longitude are 

 taken as hase stations, to which intermediate observations 

 are referred. The total force of the Earth's magnetism, 

 expressed in the scale in which the British units already 

 referred to are employed, has been found to vary at dif- 

 ferent points of the Earth's surface where observations liave 

 hitherto been made, from about 6-4 to 15-8. Before the 



practice was adopted of determining absolute values, va 



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rious relative scales were employed, not always commen- 

 surable with each other. The one most generally used 

 (and which still continues to be very frequently referred to), 

 was founded on the time of vibration of a magnet observed 



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sent centurv, at a station in South America where the 

 direction of tlie dipping-needle was horizontal ; a condi- 

 tion which was for some time erroneously supposed to be 

 an indication of the minimum of magnetic force at the 

 Earth's surface. From a comparison of tlie times of vibra- 

 tion of M. de Humboldt's mamet in South Am.erica and 



in Paris, the ratio of the magnetic force at Paris to what 



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