143 



that the compass points correctly when the ship's head is N.E., S.E., 

 S.W. or N.W. ; then the compass will be correct in all positions of 

 the ship's head, and in all magnetic latitudes. 



When the disturbing iron of the ship is at the same level as the 

 compass, the correction is stated to be much more simple, it being 

 then only necessary to introduce a single mass of iron at the star- 

 board or larboard side, and at the same level as the compass. 



It is farther remarked that if one mass of iron is placed exactly 

 opposite another equal mass, both in azimuth and in elevation, it 

 doubles its disturbing effect : if one mass be placed opposite the 

 other in azimuth, but with elevation instead of depression, or vice 

 versa, it destroys that term of the disturban'^e which depends on sin 

 A, and doubles that which depends on sin 2 A : and if one mass 

 be placed at the same level as the compass, its effects may be de- 

 stroyed by placing another mass at the same level, in an azimuth 

 differing 90° on either side. If a disturbance, from whatever cause 

 arising, follow the law of + sin 2 A, (changing sign in the success- 

 ive quadrants, and positive when the ship's head is between N. and 

 E.), it may be destroyed by placing a mass of iron on the starboard 

 or larboard side at the same level as the compass ; if it follow the 

 law of — sin 2 A, the mass of iron must be on the fore or aft side. 



From the consideration of the expression for the disturbing forces 

 produced by the ship, it is farther inferred, that both in the con- 

 struction of the ship and in the fixing of correctors, no large mass of 

 iron should be placed below the compass. 



The expressions for the disturbing forces towards north and east, 

 being transformed into forces towards the ship's head and towards 

 the starboard side, give 



I cos ^. (— M + P) cos A + I sin ^. N, for the former, and 



I cos ^. (M + P), for the latter. 



The author next proceeds to investigate the effects which result 

 from the combination of induced magnetism with permanent mag- 

 netism. Calling H, S and V the new forces arising from the latter, 

 and directed towards the ship's head, its starboard side, and verti- 

 cally downwards, the whole disturbing force towards the ship's 

 head becomes 



H + Icosc\(-M + P)cosA + Isin^.N; 

 and the whole disturbing force towards the starboard side, 

 S 4- I cos d. (M + P) sin A. 



The manner in which the numerical values of these quantities may 

 be found from experiment is then pointed out, and being determined 

 from the observations on board the Rainbow, at Station I., a compa- 

 rison is made between the observed disturbances of the needles, and 

 those which would result from the action of the ship as a permanent 

 magnet. From this comparison it appears that almost the whole 

 disturbance is accounted for by the permanent magnetism, and that 

 the residual part follows with sufficient approximation the law of 

 changing signs at the successive quadrants. For the complete veri- 

 fication of the theory it remained only to effect an actual correction 

 of the compass. This was done by placing below^ the compass, in a 



