539 



reckoned positive when the north point of the needle deviates to the 

 east, is given by the following expression : 



Deviation (S) = A + B sin % + C cos $ + D sin 2 f + E cos 2', 



being the azimuth (by compass) of the ship's head, reckoned 

 from the magnetic north towards the east ; 



A, D, E being constant coefficients depending only on the amount, 

 quality, and arrangement or position of the iron in the ship : B and 

 C, coefficients depending on these, and also on the magnetic dip and 

 horizontal intensity, are each consisting of two parts ; one caused by 

 the permanent magnetism of the hard iron, the deviation produced 

 by which varies inversely as the horizontal force at the place ; and the 

 other, caused by the vertical part of the earth's force inducing the 

 soft iron in the ship, the deviation produced by which varies as the 

 tangent of the dip : B representing that part of the combined attrac- 

 tion acting in a fore-and-aft direction, C that acting in a transverse, 



or athwart-ship direction. 



->C 

 From the equation tan g, the direction of the ship's force, and 



V B 2 + C 2 j the total magnetic force of the ship in proportion to the 

 horizontal force at the place of observation is obtained : for con- 

 venience, 1000 has been adopted to represent the value of the earth's 

 horizontal force at the English ports of observation, in order, by an 

 easy comparison, to note the changes on foreign stations. 



By comparison of the coefficients of the several descriptions of 

 ships, it is observed that in wood-built steam-vessels, the coefficients 

 B and C vary nearly as the tangent of the dip ; from whence it may 

 be inferred, as a general rule, that in steam machinery permanent 

 magnetism bears but a small proportion to induced ; but in iron- 

 built ships, B and C generally vary more nearly as the inverse hori- 

 zontal force, showing that they depend more on the permanent 

 magnetism of the iron of the ship, and thus confirming the view of 

 the Astronomer Royal, given in his earliest deductions (Phil. Trans. 

 1839), that the effect of transient induced magnetism is in these 

 ships small comparatively. Numerous examples are given in detail 

 of this permanency of magnetism, as also of the gradual diminution 

 of the ship's force resulting from time. 



An investigation of the coefficient D, which is caused entirely by 

 the horizontal induction of the soft iron in the ship, and which is 



