1865.] 



Character of Armour- Plated Ships. 



115 



deviation, viz. the " constant," the " semicircular," and the " quadrantal" 

 of X, or the proportion of the mean force to north on board to the force to 

 north on shore of p, the proportion of the vertical force on board to that 

 on shore, of x the heeling coefficient to windward ; also of the several con- 

 stituent parts of these coefficients. 



The following are the principal conclusions derived from these values r 1 



The introduction of armour plating, and the great increase in the amount 

 and thickness of iron used in the construction of modern ships of war, 

 have greatly increased the amount of the deviations previously considered, 

 and have given importance to two sources of error not previously consi- 

 dered, viz. the diminution of the directive force, and the heeling error. To 

 determine these, observations of horizontal and vertical force are necessary, 

 and they are now part of the regular series of observations made by the 

 Superintendent of the Compass Department in the iron-built ships of the 

 Royal Navy. 



For the formulae by which the reductions are made, and which are 

 derived from Poisson's General Equations, reference is made to the ' Ad- 

 miralty Manual for ascertaining and applying the deviations of the com- 

 pass caused by the iron in a ship,' 2nd edition, London, 18G3, edited by 

 the authors. This work has been translated into French by M. Daron- 

 deau, into Russian by Captain Belavenetz, of the Russian Navy, and into 

 German by Dr. Schaub, the Director of the Hydrographical Department 

 of the Austrian Navy. 



The observations confirm the conclusion formerly obtained, that the 

 semicircular deviation in an iron-built ship is chiefly due to the attraction 

 of the north point of the compass to the part of the ship which was south 

 in building, modified in armour-plated ships by the direction of the ship 

 while being plated. 



The observations also show the rapid changes which take place in the 

 semicircular deviation soon after launching, and the considerable changes 

 which take place for about a year afterwards, and the great permanence of 

 the semicircular deviation after that time. 



Observations are yet wanting from which the separation of the principal 

 part of the semicircular deviation B into its two constituents can be derived 

 with much certainty. The following can only be looked on as approximate. 



The great difference in the values of the last part in the ' Warrior ' and 

 ' Black Prince ' depend on this, that the ' Warrior ' was built head north, 

 the 'Black Prince' head south. 



