22 LIGHT SCIENCE FOR LEISURE HOURS. 



which dips. It is clear, therefore, that if we travel 

 from one hemisphere to the other we must find the 

 northern dip of the needle gradually diminishing, till at 

 some point near the equator the needle is horizontal ; 

 and as we pass thence to southern regions, a gradually 

 increasing southern inclination is presented. This has 

 been found to be the case, and the position of the line 

 along which there is no inclination (called the magnetic 

 equator) has been traced around the globe. It is not 

 coincident with the earth's equator, but crosses that 

 circle at an angle of twelve degrees, passing from north 

 to south of the equator in long. 3 west of Greenwich, 

 and from south to north in long. 187 east of Green- 

 wich. The form of the line is not exactly that of a 

 great circle, but presents here and there (and especially 

 where it crosses the Atlantic) perceptible excursions 

 from such a figure. 



At two points on the earth's globe the needle will 

 rest in a vertical position. These are the magnetic 

 poles of the earth. The northern magnetic pole was 

 reached by Sir J. G. Ross, and lies in 70 N. lat. and 

 263 E. long., that is, to the north of the American 

 continent, and not very far from Boothia Gulf. One 

 of the objects with which Ross set out on his celebrated 

 expedition to the Antarctic Seas was the discovery, if 

 possible, of the southern magnetic pole. In this he 

 was not successful. Twice he was in hopes of attaining 

 his object, but each time he was stopped by a barrier 

 of land. He approached so near, however, to the pole, 

 that the needle was inclined at an angle of nearly 



