38 MAGNETISM. 



Every seaman must have noticed the vibrations of his compass 

 when running before the wind, or when the ship rolls heavily 

 from side to side. I have seen the angle of vibration as much as 

 45*> ; that is two points on each side of the course. This vibra- 

 tory motion of the compass case is caused by the inversion of the 

 polarity of the guns and other cast and wrought iron articles on 

 board. To illustrate my meaning, let us suppose that there is 

 a gun on each side of the bittacle, on the quarter-deck ; when 

 the ship rolls, or is inclined to the starboard, the breech of the 

 lee gun becomes elevated and attracts the north point of the com- 

 pass towards the lee side of the ship, while the breech of the 

 weather gun becomes depressed, and attracts the south point of 

 the compa ss towards the weather side. When the inclination 

 or roll of the ship is changed, the polarity of the guns is in- 

 verted, so that the vibratory motion of the compass continues as 

 long as the ship continues to roll ; now all this may be proved by 

 experiment, and is, beyond a doubt, true. 



If a ship's head be north by compass, when perfectly upright, 

 and there be an object right a-head of her, should she become 

 inclined, say to starboard, without changing the direction of her 

 keel, the object right a-head will no longer bear north by com- 

 pass, but to the westward of it, in consequence of the north 

 point of the compass being drawn to leeward. If the inclination 

 of the ship be to port, the object right a-head would bear by 

 compass to the eastward of north. From this it is evident, that, 

 in north magnetic latitude, the north point of the compass will, 

 by the inclination of the ship, be drawn towards the lee-side of 

 the ship. If the ship's course be towards the north, she will be 

 found to leeward of her reckoning. If steering towards the south, 

 she is liable to be to windward of her reckoning, because the 

 south point of the compass is drawn towards the weather-side, 

 thereby indicating a course more to leeward than that actually 

 steered . 



The local attraction, du0 to the guns only, from the change of 

 polarity in them, occasioned by the inclination of the ship, may 

 be easily ascertained on board of any of our guard ships at their 

 moorings. Suppose the ship's head is ncJrth ; if all the starboard 

 guns be depressed 10*^, and all the larboard guns elevated 10 

 degrees, then the compass would indicate the deviation due to 

 the guns only, when the ship is on the larboard tack, and steer- 

 ing north, under an inclination of ten degrees. Now this is an 

 experiment so very easily performed, that it ought to be made 

 in a ship of each class. 



