GENERAL NAVIGATION. 23 



the annual change, as given on the compass rose, facilitate correc- 

 tions when the change has been considerable. The compasses are 

 reengraved when the error amounts to a degree. More frequent 

 alterations on one spot in a copper plate would not be practicable. 



The geographical change in the variation is in some parts of the 

 world so rapid as to need careful consideration, requiring a frequent 

 change of the course. For instance, in approaching Halifax from 

 Newfoundland the variation changes 10° in less than 500 miles. 



Local magnetic disturbance of the compass on board ship.— 

 The term " local magnetic disturbance " has reference only to the 

 effects on the compass of magnetic masses external to the ship. Ob- 

 servation shows that disturbance of the compass in a ship afloat is 

 experienced in onl}^ a few places on the globe. 



Magnetic laws do not permit of the supposition that the visible 

 land causes such disturbance, because the effect of a magnetic force 

 diminishes so rapidly with distance that it would require a local cen- 

 ter of magnetic force of an amount absolutely unknown to affect a 

 compass half a mile distant. 



Such deflections of the compass are due to magiietic minerals in 

 the bed of the sea under the ship, and when the water is shallow and 

 the force strong, the compass may be temporarily deflected when 

 passing over such a spot; but the area of disturbance will be small 

 unless there are many centers near together. 



The law which has hitherto been found to hold good as regards 

 local magnetic disturbance is that north of the magnetic equator the 

 north end of the compass needle is attracted and south of the mag- 

 netic equator repelled by any center of disturbance. 



It is very desirable that whenever a ship passes over an area of 

 local magnetic disturbance the position should be fixed, and the facts, 

 as far as they can be ascertained, reported. 



Use of oil for modifying the effect of breaking waves. — 

 Many experiences of late years have shown that the utility of oil for 

 this purpose is undoubted and the application simple. 



The following may serve for the guidance of seamen, whose atten- 

 tion is called to the fact that a very small quantity of oil skillfully 

 applied may prevent much damage both to ships (especially of the 

 smaller classes) and to boats hj modifying the action of breaking 

 seas. 



The principal facts as to the use of oil are as follows : 



1. On free waves, i. e., waves in deep water, the effect is greatest. 



2. In a surf, or waves breaking on a bar, where a mass of liquid is 

 in actual motion in shallow water, the effect of the oil is uncertain, 

 as nothing can prevent the larger waves from breaking under such 

 circumstances, but even here it is of some service. 



