2 5 6 SCIENTIFIC RECREATIONS. 



electro-magnet. To understand the science it will be necessary to mention 

 Ampere's theory of magnetism. 



It was CErsted who observed that when a magnet is placed within 

 reach of an electric current and free to move, it sets itself at right angles 

 to the direction of the current ; and Ampere defined the law already 

 referred to when treating of the electric current, viz., " that if a person be 

 imagined as placed in the wire so that the current shall pass through him 

 from feet to head, if he turn his face toward the magnetic needle the north 

 pole will always be deflected to his left-hand side." When the current is 

 passed above the needle from south to north poles, the deflection is to the 

 west ; when from north to south the deflection is east. When the current 

 is below the needle the contrary is the case. Ampere decided that currents 

 circulating in the same direction attracted each other, and when running in 

 opposite ways they repelled each other. He supposed currents to circulate 

 within all magnetic substances, and then that is, when the body is 

 magnetized these currents flow in planes parallel to each other, and the 

 material which offers the least resistance to the circulation of these currents 

 becomes the most magnetic. 



The earth being supposed to be an immense magnet has currents 

 circulating through it in a direction from east to west; and having the 

 property or power of turning a magnetized bar in a direction similar to that 

 in which the bar would be turned by a magnet, the earth is considered a 

 magnetic mass. This influence is due to what is called "terrestrial magnetism." 

 If we suspend a bar by a thread it will point in no particular direction, but 

 may be turned towards any side we please. But when once the needle is 

 magnetized it will point north and south ; or, as we say (but not correctly), the 

 north pole of the magnet points to the north of the globe. It is really the 

 south pole that points to the north, and the north pole of the magnet points 

 south> as can be proved by suspending the bar over another magnetized 

 bar. So if the earth be considered a magnet our English terms are incon- 

 sistent with our theories. Continental writers are more correct. 



The line of the magnetic needle's direction, which differs in different 

 places, is called the magnetic meridian, and the amount of its divergence 

 from the astronomical meridian is termed its declination or variation. When 

 the amount of this variation is known it is allowed for, and the needle can 

 be considered as pointing due north and south. 



But the needle does not assume a position perfectly parallel to the 

 horizon. It dips down in different hemispheres. As we approach the 

 north pole the dip or inclination will become greater, and the same effect is 

 observable at the south pole. Again, there are certain places on the earth 

 where the attraction is so evenly balanced that the needle is perfectly 

 horizontal. The line uniting these places is the magnetic equator. This 

 does not coincide with the earth's equator any more than the magnetic 

 poles coincide with the geographical poles of the earth. 



The declination of the needle varies from the meridian of Greenwich at 



