308 TERRESTRIAL MAGNETISM.—MURPHY. 
to each station and the direction in which the needle points is 
noted. Of course, if no disturbing mass was near, it would 
point to the north at every station. But an iron ship is there, 
so that at certain stations we find the needle repelled from 
the vessel. Now, only north magnetism can produce this kind 
of deflection. It varies in degree at each station, and where 
greatest there is the pole. Again we find the needle’s north 
end attracted towards the ship, hence we have discovered the 
body of south magnetism, so we locate its pole where deflection 
is greatest. Finally at several stations in an irregular path 
from bottom to rail we see that the needle points everywhere 
and this is the neutral line. A sketch of each side of the ship 
is drawn on paper, and the degree of deflection at every station 
is plotted by means of measurements from a line taken across 
her bow and from the ship’s side. 
“ An iron ship, frames, plating, decks, beams, stanchions, carlings, 
engines, masts, &e., is not like the steel bar, a simple magnet, but 
a network of magnetic entanglement, yet, however complex 
this may be, proper means are devised fur coping with it. The 
problem is simplified to pairs of parallel forces, each pair having 
its resultant parallel to the co-ordinate axis, the sum total of all 
three forces parallel to it, and the whole concentrates upon the 
north point of the compass; whence the final result that we have 
imaginary magnets, one laid horizontal to the axis of the vessel, 
the second also horizontal across the vessel, and the third vertical. 
“ 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, that we may obtain all the prime factors, 
and then reduce those factors as nearly as practicable to zero.” 
By swinging a ship at compass buoys or steaming in a circle 
on the open sea, the magnetic effect of the ship, that is of the 
three imaginary magnets, is brought to bear on every point of 
the needle causing it to deflect from the magnetic meridian by 
different angles at different points. These various deflections 
being serially arranged constitute what is known as tables of 
deviation. 
Frequently means are provided for opposing the magnetism 
of the ship by other powerful magnets, thus permitting the 
