34 Hature'0 Miracles. 



with it. This variation is uniform and in nav- 

 igation is reduced to a science, so that the mari- 

 ner knows how much to allow for the varia- 

 tion. Columbus, as heretofore mentioned, was 

 supposed to have first noticed this varia- 

 tion and it made him trouble. He did not 

 know how to account for it, and as his crew 

 thought the laws of nature were changing be- 

 cause they were so far from home he saw the 

 necessity for some sort of explanation. So, 

 like the brave man that he was, he hatched up 

 a theory that satisfied the crew, and although 

 in the light of the closing years of the nine- 

 teenth century it was a questionable one, it 

 worked well enough in practice to serve his 

 purpose. 



We have already stated that the earth was a 

 great magnet, and that probably it was an 

 electromagnet, caused by earth currents cir- 

 culating around the globe. You want to know 

 how the earth can be a magnet unless it has an 

 iron core like an electromagnet. Magnetism 

 or magnetic lines of force may be developed 

 without the presence of iron. When we pass 

 a current of electricity through a wire, mag- 

 netic lines of force are thrown out at right 

 angles with the direction of the current. This 

 will be fully explained further on. If we wind 

 the wire into a coil, or helix, these magnetic 

 lines are concentrated. If now we suspend 

 this helix, or, better, float it on water so that it 



