MAGNETISM AND THE COMPASS. 667 



reaches its extreme Westerly position at 1.10 p.m. It then returns to the 

 Eastward, but less rapidly, the Easterly Deviation becoming a maximum 

 about 10 p.m. ; the mean daily range equals 9"3'." 



The Irregular Changes or Magnetic Storms, as they have been termed 

 (see page 458), occur without any previous notice, and are of very great 

 extent ; some of them almost throughout the globe. At times this Devia- 

 tion amounts to 2°. 



The ascertained Variation of the Compass, in various parts of the North 

 Atlantic Ocean, is attached to the Tables of Positions in the former part 

 of this work, and are also given on the Chart facing page 455. 



The Dip, or Inclination. — The Dip of the needle is the angle which a 

 well-balanced needle forms with the horizon after it is rendered magnetic, 

 and when it has the power of free motion in the plane of the magnetic 

 meridian. As before stated, this angle varies in different parts of the 

 globe, being at zero on the Magnetic Equator, and 90° on the Magnetic 

 Poles. The Dip, like the Variation, undergoes a continual change, 

 increasing in some parts of the world, and diminishing in others. Thus 

 at Paris, in 1761, it was 75°; in 1829, only 67° 41'. At London, in 1576, 

 it was 71° 50' ; in 1837, it was 69° 20' ; in 1891, it was 67° 21-4'. 



The Dip is a very important element in magnetic consideration, and is 

 too much overlooked by the sailor. The instruments for its measure- 

 ment, however, are expensive and delicate, and require great nicety in 

 their management ; and for these reasons it is comparatively neglected ; 

 but as it is in some degree a measurement of the Intensity of the Magnetic 

 Force, and also greatly modifies the directive power of the Compass, it is 

 very important to the mariner. When the needle is perpendicular, as it 

 is over the Magnetic Poles, of course its directive force vanishes. The 

 diurnal change in the Dip amounts to 3' or 4', and is also about 15' greater 

 in summer than in winter. 



The Intensity. — The Intensity of the Magnetism of the earth varies also 

 with time and place. It is the power of the earth to bring an oscillating 

 needle to a state of rest ; and it is in proportion to the squares of the 

 number of vibrations per second. The lines of Equal Intensity would at 

 first seem to coincide with those of Equal Dip, but, in consequence of the 

 double Magnetic Polar axes, they differ in their relation, though they still 

 form regular and symmetrical curves. As the Magnetic Latitude increases 

 so does the Intensity, but not the Directive Force ; for, when a needle is 

 on the Magnetic Equator, it naturally preserves its horizontality, and, 

 consequently, the whole of its Magnetism is employed in directing the 

 needle towards the Poles. But, in high Magnetic Latitudes, where the 

 Dip is great, the means employed to keep it parallel with the horizon of 

 course reduce very considerably its power of keeping in a North and 

 South direction ; and in the circum-polar regions the ordinary Compass 

 becomes so sluggish as to be of but little value to steer by. 



Having thus very briefly sketched the general phenomena of Terrestrial 

 Magnetism, the reader will understand the general principles laid down 

 by Dr. Halley and Professor Hansteen, that in the Northern Hemisphere 

 the two points of convergence of the Magnetic Variation or Declination, 

 by revolving around the Pole of the earth, will cause a local change in 



