126 Astronomical and Nautical Collections* 



the other masses of iron by which it is surrounded. All these 

 bodies are magnetized by the influence of the earth ; in this state 

 they act on the compass, and cause it to deviate sometimes very 

 considerably, the deviations derived from this cause having 

 amounted to 20° on either side of the natural direction of the 

 needle, and even to near 40° in a voyage to high northern lati- 

 tudes. They vary in the same place according to the situation of 

 the vessel with regard to that direction ; they vary also when the 

 direction of terrestrial magnetism varies, \^ith the latitude ; so that 

 it was desirable to find a method of destroying them applicable to 

 all possible directions of the action of the earth, with regard to 

 lines in certain fixed positions within the vessel. The method 

 proposed by Mr. Barlow has been adopted in several voyages of 

 considerable length ; and if it has not entirely annihilated the devi- 

 ations of the needle, it has at least confined them within narrow 

 limits ; and it has hitherto been judged sufficient for the occasions 

 of navigation. It consists in placing near the compass a plate of 

 iron, which is also magnetized by the action of the earth. It is 

 placed in such a manner that the needle shall assume and preserve, 

 in all the directions of the ship, a direction parallel to that of ano- 

 ther horizontal compass, placed on shore, at a sufficient distance 

 from the vessel to be out of the reach of its magnetic influence. 

 Mr. Barlow supposes that, by trials, it will always be possible to 

 find a position for the plate which will give it this property ; and 

 when it has been determined, he fixes the plate, and keeps it al- 

 ways in the same situation. If the other masses of iron contained 

 in the vessel do not undergo considerable displacements, and if in 

 reality the deviations have been annihilated in all directions at the 

 point of departure, it is evident that they will continue to be neu- 

 tralised throughout the voyage, notwithstanding the changes of the 

 intensity and the direction of the terrestrial magnetism : and this is 

 easily seen when we consider, on the one hand, that all the bodies, 

 susceptible of being rendered magnetic, which form a part of the 

 vessel, including Mr. Barlow's iron plate, being magnetized by the 

 action of the earth, the intensities of their magnetic actions will in- 

 crease or decrease in the same proportion as this force : and on the 



