268 On the Correction of the Compass in Iron Ships. 



venient expression will probably be, the square of the number 

 of vibrations made in one minute of time), by the vibration, 

 either of the compass-needle if very finely mounted, or by the 

 vibration of a needle suspended by a silk fibre, the compass 

 being removed during this observation. 



It is almost unnecessary to say that the apparent bearing 

 of the ship's head as referred to the disturbed compass, or 

 rather the bearing of the disturbed needle as referred to the 

 head-and-stern line of the ship, is to be observed. 



The circle represented in the accompanying diagram, PI. XI. 

 (which, in practice, ought to be made from an engraving, in 

 order that a separate circle may be used for each separate cor- 

 rection of compass) is to be conceived as attached to the ship's 

 deck, with the line H S parallel to the ship's keel. 



Then the operation proceeds as follows : — 



The ship's head is to be placed in three different azimuthal 

 directions ; the most favourable will be three directions which 

 very rudely divide the horizon into three not very unequal 

 azimuths. In each direction, the magnitude of the actual 

 directive force, and the bearing of the disturbed needle as re- 

 ferred to the head-and-stern line, are to be observed. 



In the first observation, let A represent the magnitude 

 and direction of the actual directive force ; in the second and 

 third observations, let B and C represent similar quanti- 

 ties. Take the metallic T-square represented at the bottom 

 of the diagram ; apply its graduated edge to the points B and 

 C so that the numerations of the graduations touching B and 

 C are equal (the scale of the graduations is unimportant, all 

 that is required being that they be equal on the right side 

 and on the left side), and draw a pencil along the fiducial edge 

 of the transverse arm, thus describing the line be, be. Apply 

 the graduated edge in like manner to the points A and C, and 

 thus describe the line a c, ac, intersecting the former line at 

 P. The same operation may be performed on A and B, de- 

 scribing the line a b, ab ; but it is not required, as that line 

 will necessarily pass through the point P. 



Then P represents in direction and in magnitude (on 

 the same scale as A 0, B 0, C 0) the magnetic force which 

 must be introduced for the correction of the compass. For, 

 it is a force which accompanies the ship in all its motions ; 

 and its introduction, and its composition with the observed 

 forces A 0, BO, CO, will produce the resultant directive 

 forces A P, B P, C P ; which, from the nature of the geo- 

 metrical process, are equal, and will represent the terrestrial 

 directive force, equal in magnitude for the three positions of 

 the ship. 



