210 Influence of Terrestrial Magnetism on Iron. 



Navigators also are exposed to making great errors, which 

 might be attended with serious consequences. Suppose, for 

 example, that the axis of the vessel, that is to say, the line 

 going from stem to stern, was at first perpendicular to the 

 plane of the magnetic meridian, and directed to the west ; 

 that, in this position, the deviation of the needle was 20° to the 

 west of the direction that it ought to have ; a change in the 

 course of the vessel causes the axis to turn 180°, namely, from 

 west to east ; by the effect of this change of direction, the de- 

 viation has also passed from west to east, and is conse- 

 quently 20° to the east. It is evident that the observer, who 

 should not be acquainted with the action of the iron contain- 

 ed in the vessel, to which these two deviations of 20°, first to 

 the west and then to the east, are due, would believe that 

 the needle has remained parallel to itself, and would judge 

 that the rotation of the vessel had only been 180°— 20° + 2, 

 or 180°- 40°, namely, 140°, whilst it had really been 180°. 

 He would have been deceived, therefore, to the amount of 

 40° on the second direction of the vessel, supposing that he 

 had carefully determined the first direction by the ordinary 

 processes. 



Mr Barlow proposed various means of avoiding the dan- 

 gerous errors to navigation that we have just pointed out. 

 One of these means consisted in placing in the neighbour- 

 hood of the compass a plate of soft iron, which becomes mag- 

 netised like the other masses of iron in the vessel, by the in- 

 fluence of the globes. This plate is put into such a position 

 in front of the compass, that its action upon the needle shall 

 be exactly equal to the total action of all the iron distribut- 

 ed throughout the vessel ; so that, by removing the plate, 

 one half of the local deviation is removed, whence the amount 

 of local deviation due to the ship's iron is readily obtained. 

 The position that should be given to this plate has been 

 previously determined by trials. 



Another means has also been employed by Mr Barlow, 

 from numerous experiments made by placing the vessel in 

 every azimuthal position, and comparing, by means of two 

 telescopes, the direction of its compass in every position with 

 that of a magnetised needle remaining on the shore. Pie sue- 



