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The following communication was read : 



" On the Effect produced on the Deviation of the Compass by 

 the Length and Arrangement of the Compass Needles; 

 and on a New Mode of correcting the Quadrantal Devia- 

 tion." By ARCHIBALD SMITH, Esq., M.A., F.R.S., late 

 Fellow of Trinity College, Cambridge; and FREDERICK 

 JOHN EVANS, Esq., R.N., Superintendent of the Compass 

 Departmentof Her Majesty's Navy. Received April 13, 1861, 



(Abstract.) 



When the length of the compass needle may be neglected com- 

 pared with the distance of the iron which acts on a ship's compass, 

 the deviation is accurately expressed by the formula 



in which is the azimuth of the ship's head measured eastward from 

 the correct magnetic north ; 



' is the same azimuth, but measured from the direction of the 

 disturbed needle ; 



= $ is the easterly deviation of the needle ; 



A, D, E are coefficients depending on the distribution of the soft 

 iron of the ship. 



B and C are coefficients depending partly on the distribution of the 

 hard and soft iron of the ship, and partly on the magnetic dip and 

 horizontal force at the place. 



In all ships which have been examined, A and E are so small that 

 they may be neglected ; and, if the deviation be of such an amount 

 that we may take 3 for sin 3, 



a=B sin ' + C cos ' + D sin (+') 



The first two terms represent the " Semicircular" deviation, the 

 last term the " Quadrantal" deviation. 



The "semicircular" deviation is, on the plan proposed by the 

 Astronomer Royal, and extensively practised in the mercantile marine, 

 corrected by magnets ; the "quadrantal " by masses of soft iron placed 

 on each side and at the same level as the compass ; and when the di- 

 stance of the correction is sufficiently great, this correction may be 

 considered as perfect for the time and place at which it is made ; but 

 when this is not the case, errors are introduced, which it is the object 

 of the paper to consider. 



Mr. Evans observed that the standard compass ot the 'Great 



