116 Messrs. Evans and Smith on the Magnetic [Mar. 16, 



In the iron-built armour-plated ships the quadrantal deviation becomes 

 very large, very much exceeding what has been found in other ships. This, 

 however, is not to be attributed in all cases to the armour-plating, as is 

 shown by the small values of the quadrantal deviation in the wood-built 

 armour-plated ships ; and theory as well as observation shows that, in the 

 case of a compass in a central position, the armour-plating rather tends to 

 diminish the quadrantal deviation. The different amount in the different 

 ships is completely accounted for by the position of the bulkheads and 

 armour-plating, rifle-towers, &c. 



Thus in the following cases, in which the position of the armour-plat- 

 ing relatively to the compass is such as to increase the quadrantal devia- 

 tion, the values are, 



In the following cases of iron-built ships, the armour-plating is so placed 

 as to have little effect on the quadrantal deviation. 



In the following wood-built armour-plated ships, the armour-plating 

 being carried all round, and the compasses near the centre of the vessel, 

 the effect is to diminish the quadrantal deviation. 



In one of the turret ships, where the compass was out of the midship 

 line, E, the other part of the quadrantal deviation, attained a large amount, 

 being 4 for one compass, and 9 for the other. In all the cases of com- 

 passes in the midship line, E is small. 



The diminution of the directive force in these ships is also remarkable. 

 In the main-deck compasses of some of the iron-built armour-plated ships 

 the mean directive force hardly exceeds ^ of that on shore. 



The most remarkable feature, both in the quadrantal deviation and in 



