THE COMPASS IN MODERN NA Via A TION -.m 



the varying amount of the horizontal component of the intensity 

 of the terrestrial magnetic field, and a variation of the compa.ss 

 of larger or smaller amount according to geographical position— 

 the ship herself would exert no intluence wliatever. Hut, in 

 modern navigation, instead of guiding a vessel having no mag- 

 netic influence whatever over the glohe — a great magnet whose 

 magnetic elements are known— the mariner's compass is em- 

 ployed in guiding a steel vessel, which is a great magnet, whose 

 magnetic elements are ever varying and capricious, over the 

 glohe, a greater magnet. 



If a bar-magnet bel)rought into a horizontal position un<Ier a 

 compass-needle that has assumed a steady j)osition under tlie 

 influence of the earth's magnetism, the compass-needle will im- 

 mediately move and assume a position which is the resultant of 

 the joint action of the earth and the bar-magnet ; and with cvcrv 

 change in the azimuth or inclination of the bar-magnet the com- 

 pass-needle will assume a new resultant position. This is anal- 

 ogous to the joint action of the magnetism of the earth and the 

 iron s^ip on the mariner's compass, only the influence of the 

 ship is vastly complicated by the existence, along with her per- 

 manent magnetic elements, of the ever-varying magnetic eflect*; 

 resulting from the inductive action upon the ''soft" iron of the 

 ship, of the fields of the earth's magnetism, and the ships per- 

 manent magnetism. 



If a C3'linder of pure Avrought iron that has not been hannncrcd 

 and is entlrel}^ free from magnetism be held vertically in our 

 latitude the upper end instantly becomes a south an<i tlie lower 

 a north pole. If it be reversed, the magnetism also reverses, so 

 that tiie upper and lower ends are still as they were l)efore — a 

 south and a north pole, respectively. When it is held horizontally 

 in the meridian the end toward the north becomes a north i><>le, 

 while that toward the south becomes a south pole ; and when 

 it is revolved slowly or rapidly in azimuth, the foci of niagnetic 

 polarity move with the fidelity of a shadow, until when the cyl- 

 inder points east and west, all the side lacing the north is per- 

 vaded by north magnetism, and all facing the south by south 

 magnetism. Again, let us conceive the hull of a ship to l)e like 

 the cylinder of ))ure wrought-iron and as susceptible of mag- 

 netic induction in being steered over its ever-changing courses 

 as the cylinder is when turned into difierent positions. Then, 

 as the ship steers north, in the northern magnetic hemisphere, 

 the bow will become the center of north polarity and the stern 



