8io THE POPULAR SCIENCE MONTHLY. 



and to the south of it in the western hemisphere. It further ap- 

 pears that the magnetic force is not evenly distributed on the 

 earth, and that the points of maximum intensity do not coincide 

 with either of the magnetic poles. In the northern hemisphere 

 there are two foci of maximum force of unequal intensity, the 

 most powerful lying at about latitude 53° north, longitude 92° 

 west, near the great American lakes, the weaker in latitude 65° 

 north, longitude 115° east, in Siberia. For the southern hemi- 

 sphere, the available data are far less numerous, and the deter- 

 mination of the foci of force is less reliable. It is, however, be- 

 lieved that here also there are two points of maximum, of nearly 

 equal power, and not far removed from one another, one in lati- 

 tude 65° south, longitude 140° east, the other in latitude 50° south, 

 longitude 120° east. The unit by which magnetic force is meas- 

 ured has been assumed, adopting English standards of weight 

 and length, to be that which would impart to a weight of one 

 grain a velocity of one foot in one second of time. On this scale 

 the magnetic force, where least, is found to be 6'0 ; the northern 

 maxima are 14'2 and 13*3 respectively, and each of the southern 

 15*2. The declination, or variation of the direction of the needle 

 from the true meridian, is a consequence of these unequal forces 

 operating upon it, the westerly or easterly tendency of the needle 

 (as the case may be) following the geographical position of the 

 place of observation in its relation to the several foci of force, 

 with a general result of considerable complexity. Up to the six- 

 tieth parallel of latitude, north or south, the declination, whether 

 easterly or westerly, rarely exceeds 30° ; and, speaking generally, 

 it is easterly in the Pacific and westerly in the Atlantic and Indian 

 Oceans, Near the poles, where the dip becomes high, the directive 

 force of the earth's magnetism becomes much reduced, and the 

 magnetic needle becomes comparatively unreliable and of little 

 use. The nature and mode of operation of magnetism, and the 

 allied phenomena of electricity, continue to be subjects of specu- 

 lation, no explanation of them having yet been proposed, such as 

 that which refers heat and light to the vibrations of an elastic 

 medium. Our knowledge of the phenomena of terrestrial mag- 

 netism therefore still remains in the empirical stage ; they are, 

 however, held to show that the earth's magnetism is distributed 

 through its mass, and that the magnetic force either wholly or 

 mainly resides in the interior, and can not be attributed to exter- 

 nal influences, though it may be affected by them. Whether or 

 not geographical features have any influence on the distribution 

 of this force is doubtful. Observation shows that all the elements 

 of the earth's magnetism not only vary from place to place, but 

 from time to time ; the variations being in some cases periodical 

 and dependent on the time of the day or the season of the year. 



