MAGNETISM. 423 



wiiat Is an nieans of a soft iron bar called an Armature, 

 Armature? represented at A, Fig. 349. 



This becoming magnetic by induction, reacts upon the 

 magnetism in tlie poles of the magnetic bar, and tends to 

 increase rather than diminish their intensity. 

 What is the The lifting or sustaining power of magnets 

 fichama^ete'f varics very materially. The most powerful 



that we are acquainted with are capable of 

 sustaining twenty-six times their own weight. 

 How does the Thc law of magnetic attraction and repul- 

 noucaura^ron ^^0^ ^^ thc samc as that of gravitation ; that 

 Tary ''^'^^"'""'^ is, tlicsc forccs iucrcase in the same proportion 



as the square of the distance from the center 

 of attraction or repulsion diminishes. 



. ,. ^ 795. The various phenomena of roatmetisra have been ac- 



Accorairg *" 



▼hat theory are Counted for by supposing that all bodies susceptible of magnet- 



Boinena° ^ac- ^^™ ^^® pervaded by a subtle imponderable fluid, which is corn- 

 counted for? pound in its nature, and consists of two elements, one called 

 the austral, or southern magnetism, and the other the boreal, 

 or northern magnetism. Each of these, like positive and negative electrici- 

 ties, repel their own kind, and attract the opposite kind. 



"When a body pervaded by the compound fluid is in its natural state and 

 not magnetic, the two fluids are in combination and neutralize each other. 

 When a body is magnetic, the fluid which pervades it is decomposed, the austral 

 fluid being directed to one extremity of the body, and the boreal to the other. 



Iron and steel are easily rendered magnetic, because the fluids which per- 

 vade them can be easily decomposed by the action of other magnets. In 

 iron, the separation of the two kinds of magnetism may bo easily, but only 

 transitorily effected. The magnet, therefore, attracts it powerfully, convert- 

 ing it, however, into only a temporary magnet. In steel, the two kinds of 

 magnetism are not so easily separated ; hence the latter is but slightly at- 

 tracted by the most powerful magnets. "When once effected, however, the 

 separation is permanent, and the steel becomes a perfect magnet. 



As, according to this theory, the act of rendering a body magnetic consists 

 simply in decomposing a fluid pervading it, we can easily understand ho^\; 

 by means of one artificial magnet, an infinite number of other magnets may 

 be made, without the former losing any of its magnetic properties. 



^,t i, ^ 796. The Magnetic Needle (Fig. 353) is 



Magnetic Nee- gimply a bar of steel, which is a magnet, bal- 

 anced upon a pivot in such a way that it can 

 turn freely in a horizontal direction. 



