THE COMPAHS IN MODERN NA Vl(i.\ TlOX iv,; 



Throughout the liistory of oce:m luivi^Mtion it lias reinained pre- 

 eminent iiinont,' nautical instruinents; and to(hiy, l.y tli,. Hide of 

 the chronometer and sextant, it is scarcely less important than 

 it was when it constituted the navigator's sole eciuipnient. The 

 later instruments have contrihuted to precision in the use of the 

 compass and to precise navigation in general, hut they have in 

 no sense supidanted it or greatly afVected the degree of its funda- 

 mental im})ortance. 



Up to the era of iron ships the management of the mariner's 

 compass was as sim[)le as the surveyor's, heing influenced hy the 

 earth's magnetism alone ; but with the growth of the application 

 of steam propulsion to modern ships and the employment of iron 

 and steel in their construction it was found that every shi|) her- 

 self becomes a great magnet like the earth is, although of lesser 

 intensity. 



It has long l)een known that the earth acts upon the magnetic 

 needle somewhat as a bar magnet does, and that it has delinite 

 poles of magnetic strength and a magnetic field surrounding it 

 Avhich may be represented in general by lines of magnetic in- 

 tensity issuing from one pole anil proceeding to the other by 

 curved paths to which a freely suspended magnetic needle will 

 ever3Mvhere set itself tangent. For more than a century it lias 

 been customary among geomagneticians to represent the elements 

 of the direction and intensity of the earth's magnetism as mani- 

 fested at its surfiice by lines conceived to be drawn upon the 

 surface of the globe. The lines passing through all places where 

 the angle between the plane of the astronomical meridian and 

 the vertical plane passing through a freely sus[)ended magnetic 

 needle is the same are called lines of equal magnetic declination 

 or, among mariners and surveyors, lines of e(]ual variation of the 

 compass. These lines issue from one n)agnetic pole and pass by 

 curved paths to the other and through the geographical poles of 

 the earth. The lines which are conceived to l)e drawn through 

 all places where the angle l)etween the direction of a freely sus- 

 pended needle and the plane of the hori/on is the same are called 

 lines of eciual magnetic inclination or dij). They gird the earth 

 in circumferences parallel to tlie magnetic equator, .somewhat the 

 same as the parallels of latitude with reference to the geo- 

 graphical eciuator. The magnetic eciuator is the line passing 

 through every i)oint at which the freely suspended needle lies in 

 a horizontal plane. As we travel from the magnetic equator to- 

 ward the northern magnetic pole the needle inclines ni..n- and 



