268 THE COMPASS IN MODERN NA VIGA TION 



more, the north end tending downwards until the pole is reached, 

 when the needle assumes a vertical direction. As we travel 

 toward the southern magnetic pole the same takes place with 

 the south end of the, needle. 



Similar results may be obtained by carrying a small needle 

 through the magnetic field of a bar-magnet. At the neutral band 

 it will be parallel to the bar, while, as either end is approached, 

 the dip toward the Pole becomes more and more ; and as with 

 the bar-magnet, which has a magnetic field that varies in inten- 

 sity from point to point, so with the earth, whose magnetic field 

 is powerful near the Poles and steadily moderates in strength as 

 the magnetic equator is approached. There is thus a third set 

 of lines passing through all points where the magnetic intensity 

 is the same. These are known as isodynamic lines or lines of 

 equal magnetic intensity. In general contour they follow the 

 lines of equal inclination or dip. 



These different systems of lines representing the magnetic ele- 

 ments have not on the earth that symmetry and regularity which 

 they would present around a steel bar ; but, on the contrary, they 

 often pursue serpentine courses with many a bend and loop ; 

 and since the values of the magnetic elements are not fixed either 

 as to time or locality, they shift their positions hourly, daily, 

 monthly, yearly, and through centuries. These changes are all 

 believed to be periodic and, with the exception of the secular 

 change, are of such small amplitude that they do not affect the 

 use of the compass on the seas where commerce is carried on. 

 So that for purposes of navigation, the terrestrial magnetic lines 

 may be drawn so as to hold good for several years from a given 

 epoch. 



A freely suspended magnetic needle dipping, as it does, every- 

 where except on the magnetic equator, is of no value to guide a 

 ship. The compass needle must be horizontal. This condition 

 is attained in practice by putting a small sliding counterpoise on 

 the needle to overcome the downward pull of the earth's mag- 

 netism, or by floating the compass-card in a mixture of water and 

 alcohol. It is, therefore, only the horizontal component of the 

 earth's magnetism that gives steadiness to the needle of the 

 compass and influences its direction. 



If a wooden ship, with no metal other than the copper in her 

 frame, were to sail around the world, her compass would experi- 

 ence only those magnetic phases that result from the influence 

 of the earth's magnetism — more or less steadiness, according to 



