666 MAGNETISM AND THE COMPASS. 



nearly equal power, exist in the Southern Hemisphere, one in lat. 65° S., 

 long. 140° E., and the other in lat. 50° S., long. 120° E. 



By the action of the dipping-needle, the line of Force is found to be 

 horizontal only in Equatorial and Tropical regions, and to lie below or 

 above the horizontal plane when to the North or South of the Magnetic 

 Equator. The direction taken by the Compass needle invariably leads to 

 the Magnetic Poles, around which the lines of Equal Dip are arranged in 

 a series of circular or elliptical curves. The maximum Magnetic Force is 

 not coincident with the Magnetic Poles, nor lines of Equal Force with 

 lines of Equal Dip. 



The Magnetic Poles, or points on the earth's surface over which the 

 dipping-needle would stand vertical, are separated by a Magnetic Equator, 

 which is not coincident with the earth's Equator, but is an irregular 

 circle, which crosses it in two points, one in mid-Pacific, and the other 

 in 12° W. in the Atlantic; on this circle, of course, the dipping-needle 

 remains horizontal. 



Bespecting the North Atlantic Ocean, we may here state that in 1895, 

 the Magnetic crosses the Terrestrial Equator in about long. 12' W., and 

 proceeds south-westerly, across the Atlantic, to the coast of Brasil, which 

 it touches in lat. 14° S. The line of Equal Dip, at 70°, runs from the 

 N.W. coast of Ireland, curving to the W.S. W., to about Washington, U.S.; 

 between these lines the lines of Equal Dip (or Isoclinal Lines) form nearly 

 regular divisions, as will be seen by referring to the diagram, page 455. 



The Mariner's Compass, as generally used, exhibits the direction of 

 the Magnetic meridian only; and in treating of the magnetic needle, for 

 the purposes of navigation, three points are to be inquired into — these 

 are, the Variation, or Declination; the Dijp, or Inclination; and the 

 Intensity of the Magnetic Force. 



The Declination, or Variation. — With this branch of the subject every 

 sailor must be perfectly familiar, and any comment on its actual state is 

 therefore unnecessary. It is described as positive when Westerly, and 

 negative when Easterly. But this Variation is not constant. There are 

 several elements of change in this part of the Magnetic Force, for it 

 undergoes Secular, Annual, Mensual, Diurnal, and also irregular Changes. 

 The Secular Change is a progressive alteration observed in the direction 

 of the magnetic needle during a series of years. Thus, in 1576, Kobert 

 Norman found the Compass in London to point 11° 15' East of North; in 

 1658, it pointed true North ; it was on the increase in 1819, when it was 

 24° 41' West of North ; and since then it has been retrograding, and in 

 December, 1891, was 17° 20' 30". 



The Mensural Change is according to the season of the year. It was first 

 noticed by Mr. Canton about the year 1756. It amounted, in January, 

 to 7' 8" ; in March, 11' 17"; in June, 13' 21"; in September, 11' 43"; and 

 in December, 6' 58". 



The Diurnal Change is thus described from the observations of Pro- 

 fessor Lloyd : — " The mean daily curve of the Changes of Declination for 

 the entire year exhibits a small Easterly movement of the North end of 

 the magnet during the morning hours, which reaches its maximum about 

 7 a.m. After that hour the North end moves rapidly Westward, and it 



