COMPASS IN IRON ARMOUR-PLATED, IRON, AND COMPOSITE SHIPS. 625 
generally of widely different forms. The values of s/ P~ + Q 3 are, therefore, chiefly 
dependent upon the position which the standard compass occupies on board these ships, 
considered as magnets. 
Again, each iron body introduced during equipment into that great magnet, the 
ship, tends to modify in one direction or another its action upon the compass. 
Consequently, if the compass be moved from the stern along the central longi¬ 
tudinal line of a ship towards her bow, it will be subjected to the influence of forces 
varying from those of repulsion or attraction, to zero, and then to those of attraction 
or repulsion. 
For example, the “ Iron Duke ” and the “Triumph ”—although not sister ships— 
are alike in many points, and built nearly in the same direction. 
- -j Distance of standard 
fP't Q"* compass from stern. 
Iron Duke ... ’416 81 feet. 
Triumph . . . "033 105 ,, 
It might at first sight be inferred from these results, that the position of the 
compass in “ Triumph ” is better than in the “ Iron Duke.” Keeping in view the 
object—always much desired—of so placing the compass as to have, when corrected, 
small changes of deviation on change of magnetic latitude, it will be seen this is not 
the case. 
A reference to the values of \/c 3 +/ 3 , representing the changing part of the 
deviation, shows that in the “Iron Duke” the value is +'060 ; in the “Triumph” 
it is +T06. Thus, the “ Triumphs” compass would be improved as regards deviation 
by moving it further towards the stern and away from the armour-plated bulkhead 
causing the large value of c, and opposing the increased value of P which would 
ensue, by a bar magnet. 
These considerations tend to show the importance of the long-established regula¬ 
tions with regard to the placing of the standard compass in ships of the Poyal Navy, 
which provide, that the best possible position with regard to surrounding iron shall be 
selected for it, subject to the interests of the ship as an engine of war. 
General Conclusions . 
The following general conclusions have especial reference to the Standard Compass 
positions in those vessels mentioned in the tables, and to all others of similar types. 
1. A large proportion of the semicircular deviation is due to permanent magnetism 
in hard iron. 
