3o6 THE POPULAR SCIENCE MONTHLY. 



The problem is now simplified to pairs of parallel forces, each 

 pair having its resultant parallel to one of the co-ordinate axes ; 

 and the effect of every magnetic particle, whether of permanent or 

 transitory magnetism, may be reduced to this condition, We may 

 now with facility transfer into each co-ordinate axis the sum total 

 of all the forces parallel to it, and concentrate the whole upon the 

 north point of the compass, whence the final result, that we have 

 reduced the entire magnetic power of the ship to that of three 

 imaginary magnets — one laid horizontally in the axis of X; the 

 second, also horizontal, in the axis of Y; and the third, vertical, 

 in the axis of Z. 



The individual and combined effect of these three imaginary 

 magnets is the object of investigation ; but, before entering upon 

 it, it will be necessary to remark that each is not simple, but 

 complex, and that, recognizing this, we shall have to consider 

 all the component parts, leaving to every real case to determine 

 which of the components reduce to zero, and which are prime 

 factors. 



The iron of a ship is of varied quality, from the " hard," which 

 when hammered acquires and keeps its magnetism, to the " soft," 

 which has absolutely no retentive power. It occupies every con- 

 ceivable direction — vertical, longitudinal, transverse, and inclined 

 at diverse angles ; but, however varied the latter, it may be repre- 

 sented in the first three directions by pieces of equivalent effect. 

 Finally, it may be symmetrical or unsymmetrical. To cover all 

 the conditions of the problem, we shall choose representatives of 

 quality and direction, of symmetry and singularity, and let each 

 assert its power in the common struggle. 



Fig. 16 represents the arena of these forces ; they are arrayed 

 in lines of attack upon the compass. 



P, Q, and R, represent hard iron, whose magnetism, the result 

 of percussion, is of a permanent nature, like that of a steel bar; 

 the hull itself of the ship is an example of this kind. 



c,f, and h represent vertical soft iron; it becomes magnetic 

 through the inductive agency of the earth's vertical force; c 

 might represent the smoke-pipes; /, boat-davits; and Tc, stan- 

 chions on the deck below that on which the compass is located. 



a, e, and g represent horizontal soft iron, the first and last, 

 when in a longitudinal direction, and e, in a transverse direction ; 

 the power of this iron is derived from the inductive agency of 

 the horizontal force of the earth ; as examples of a may be cited 

 the engines, boilers, and water-tanks ; of e, a deck -beam cut amid- 

 ships for a hatch or any other purpose ; and of g (when below 

 the compass), the shaft. 



6, d, and h are substitutes for an isolated mass, like T, that has 

 no counterpart on the opposite side, and they proclaim Ts influ- 



