THE 



POPULAR SCIENCE 

 MONTHLY. 



JANUARY, 1889. 



THE GUIDING-NEEDLE ON AN IRON SHIP. 



By LiETTTENAiiT-CoMMAirDEB T. A. LYONS, U. S. Navy. 



THERE is an agency that pervades the earth and is peculiarly 

 resident in all its iron. It is magnetism. This force is akin 

 to electricity, though not identical with it, and the manifestations 

 of both are often similar. 



The small steel wire, scarcely larger than a sewing-needle, 

 which constitutes the mariner's compass — every iron vessel, even 

 the huge steamship City of New York, and the earth itself — all 

 have certain properties in common that warrant classing them as 

 magnets ; and, as the ship sails the earth and is guided by the com- 

 pass, there is a very intimate though varying relationship between 

 these three that should deeply interest those who traverse the 

 ocean. To describe this relationship, its contentions, and the 

 constant struggle of each member for mastery, rather than their 

 amicable companionship, is the object of this article ; and it will 

 render our ideas of the subject clear if we begin by stating the 

 properties of the ordinary bar-magnet. The needle, the ship, and 

 the earth are but magnets of different size. 



The Steel Bar-Magnet. — Fig. 1 represents a steel bar which 

 has been magnetized. Its centers of power are located close to 

 each extremity, while near the middle is a neutral ground over 

 which the influence of neither end predominates. If fine iron fil- 

 ings be sprinkled around the magnet, they will form into curved 

 lines emanating from each center, and eventually trending toward 

 a union. 



These centers are called poles. The magnetism in one is oppo- 

 site in kind and equal in degree to that in the other ; there is a 

 mutual attraction between these opposite magnetisms, and this 

 tendency to rush across the neutral ground, and, by combining, 



VOL. XXXIV. — 19 



