MAGNETISM OF THE EARTH 39 



bus. We know now that the reason for the declination 

 of the needle is that the north end of it does not point 

 toward the north geographical pole as was at first supposed, 

 but toward a point in the southwestern part of Boothia 

 Felix which is called the north magnetic pole. The south 

 magnetic pole as recently determined is a little to the east 

 of Victoria Land. 



These magnetic poles do not remain in the same place all 

 of the time but swing slowly back and forth, so that the 

 declination changes for the same place. On account of 

 this it is necessary for surveyors, who use the compass, to 

 find out the declination each year. The annual change in 

 the United States varies from to 5 seconds. 



SUMMARY 



The ancients thought that the earth was flat ; but modern 

 scientists have proved in many ways that it is an oblate sphe- 

 roid, slightly flattened at the poles and bulging at the equator 

 somewhat resembling an orange in shape. Its polar diam- 

 eter is 7900 miles ; its equatorial diameter is 7927 miles, and 

 its equatorial circumference is 24,902 miles. 



The rotation of the earth on its axis gives us our days, the 

 points of the compass, and our means of measuring time. 



The earth revolves about the sun once a year, not in a 

 circular, but in an elliptical, orbit. Its average distance 

 from the sun is 93,000,000 miles, but it is 3,000,000 miles 

 closer to the sun in our winter than in our summer. Since 

 the axis of the earth is inclined 231 degrees from the perpen- 

 dicular to the plane of its orbit, the northern hemisphere in 

 summer is pointed toward the sun and in winter away from 

 it. It is not closeness to the sun but directness of its ray 

 that gives us our summer heat. The inclination of the earth 



