MAGNETISM AND THE COMPASS. 675 



at sea. There are, however, strong grounds for inferring that by a judi- 

 cious position of the Compass, so as to ensure small errors while on an 

 even keel, the errors arising from a ship's Heel will be so proportionally 

 reduced, as not practically to affect the navigation of the ship in the hands 

 of a prudent seaman* 



In the Northern Hemisphere, in ships built with their heads from about 

 S.E., through North, to S.W. (the usual effect of the permanent Magnetism 

 of the ship then conspiring with that of her vertical and transverse soft 

 iron), the North point of the Compass needle, assuming the Compass to 

 be on the upper deck and on the after portion of the ship, will, as a general 

 rule, be drawn to windward, or to the high side of the ship ; the nearer 

 the ship's head was to North whilst building, the greater the error that 

 may be expected ; the effect being to throw the ship to windward of ber 

 supposed position when steering on Northerly courses, and to leeward (m 

 Southerly courses, the error decreasing as the Equator is approached, and 

 small, perhaps of a contrary name, in the Southern Hemisphere. 



In ships built with their heads from about S.E., through South, to 

 S.W. (the usual effect of the permanent Magnetism, and that of the trans- 

 verse, &c., soft iron being then contrary to each other) the North end o( 

 the Compass needle may be drawn to leeward, depending upon whether 

 the vertical force of the permanent Magnetism, or that of the induced 

 Magnetism of transverse and vertical soft iron, predominates, but the 

 error would then, as a general rule, be small in these latitudes, and large 

 in the Southern Hemisphere. 



The Heeling error may be small or large, depending greatly upon the 

 position of the Compass — it has been known to exceed 2° for every degree 

 of Heel of the ship — and is directly proportional to the amount of Heel. 

 Consequently, if the error at North or South for 1° of Heel is known, 

 the error for any other direction of the ship's head, and amount of Heel, 

 can be found by the usual methods. 



Serious changes, however, frequently take place in the character and 

 amount of the Heeling error as the ship changes her Magnetic Latitude. 

 In the first place, from the vertical force arising from the permanent 

 Magnetism of the ship, the error from which is greatest in high latitudes, 

 diminishes until the Equator is reached, where it is least, and increases 

 again in the opposite Hemisphere, but still retaining the same name ; and 

 in the second place, from vertical induction in vertical soft iron, and 

 transverse soft iron generally (such as beams, &c.), the error from which 

 is greatest in high latitudes, diminishes until the Equator is reached, 

 where it becomes nil, and increases again (but of an opposite name) a* 

 the vessel recedes from the Equator in the opposite Hemisphere. It is 

 difficult, therefore, to predict with any great accuracy the change that will 

 take place, observations at every possible opportunity being the only 

 reliable safeguard. 



Mariners are further warned that the adjustment of Compasses by 

 magnets, soft iron, &c., which is for bringing the error within manageable 



• " Reduction and Discussion of the Deviations of the Compass," by Fred. J. Evana, 

 R.N., in the "Philosophical Transactions," 1860, part ii., pages 334—358. 



