48 



TERRESTKIAL MAGNETISM. 



[Sect. II 





^■^ 



whenever the ship has so materially changed her geographical position 

 as to have altered the magnetic inclination 30° or 40°, or whenever she 

 has been refitted, or has undergone any other change which may hare 

 made a coi^siderable alteration either in the amount or in the distribu- 

 tion of her iron. It is also particularly desirable to repeat them if the 

 ship should be in a harbour where the inclination is very small or 

 where it is very large; ar-d finally^ they should be repeated without fail 

 whenever the magnetic observations made on board a ship are broueht 

 to a terminationj and before any change has been made in the iron of 

 the ship. 



5, If the positions in which the standard compass and Fox's ap- 

 paratus are used on board are not very far from each other, and if there 

 is no iron within a few feet of either of them, it will usually be found 

 in sailing vessels built of wood at least, that the effect of the ship's iron 

 is the same, or very nearly so, in the two positions. To prove this^ place 

 a second comp.,.3 (which, like the standard compass, has had its index 

 ^rror, if any, determined) in the gimball-stand of the Fox's apparatus, 

 and observe, generally, whether the two compasses agree when the ship's 



head is on the different points, and especially on the points of greatest 

 and least error. 



G. When the observations thus described have been carefully made 

 and recorded, they furnish the means of calculating approximately all 

 the corrections required to clear the magnetic observations made on 

 board the ship, in her successive passages from port to port, from the 

 effect of the iron upon the needles of the standard compass and of Fox's 

 apparatus. 



The calculations for this purpose have hitherto been made at Wool- 

 wich, by formulse derived by Mr, Archibald Smith from the fundamental 



r 



equations of M. Poisson's theory, in his ' Memoire sur ies Deviations de 



e 



a/ ' 



la Boussole produites par le Fer des Valsseanx/ 



For the full understanding of these fornmlse, it is necessary to read 

 Mr. Smith's notices printed in the accounts of the magnetic observations 

 made in the Antarccic expeditions of Sir J. C. Eoss, and of Lieutenants 

 Moore and Clerk, published in Nos. V., VI,, and Vill. of the Memoirs 

 entitled 'Contribution^ to Terrestrial Magnetism,' in the Philosophical 

 Transactions ; but it may h^ convenient to reprint here the formulse 

 which sre of most general application. In these formulae, symbols are 

 used which have the following significations: .^ is the ^tolal magnetic 

 force of the earth observed on shore; 6 the inclination observed on shore 

 by Fox's apparatus; j,' the true magnetic azimuth of the ship's head 

 counted from the magnetic North, positive when her Iiead is West of North 

 and negative when East, rj/, Q\ and C^are the same elements observed by 

 the needles of the sta^idaid com.pass and of Fox's apparatus on board ; 



S !■«» *'" 



lli 



7 



For 



the CO 

 be CO 



i 



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mp 



,,ey may ^ ^ 



jefignated by 

 v'r\ it re^"' 



, lias been o 

 values of 5 at a 

 of these values i 

 totheT;.:'-ard 



and ail .--■- Ji^ 

 "'^'t be attend. 



A= ■ 



B 



+ • 



+ 



1 



^ 



( 



+ ■' 

 + •( 



6 



D 



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E 



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 ^ 



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