CHAPTER XXI. 



MAGNETISM. 

 THE LOADSTONE MAGNETIC CURVES THE MAGNETIC NEEDLE 



THE MARINER'S COMPASS MAGNETO-ELECTRICITY. 



WE have already mentioned some of the properties of the loadstone or 

 magnet ; but as we are now about to enter more fully into the considera- 

 tions of its attributes and of the compass, etc., we will add some further 

 interesting particulars. 



Ancient writers (Pliny, Homer, and Aristotle) mentioned the 

 existence of the magnet, and Humboldt refers to the knowledge of it 

 possessed by the ancients. Pliny says " the magnet-stone is found in 

 Cantabria," and we have heard of the loadstones that are supposed to 

 support Mahomet's coffin at Medina. The origin of this fable was (pro- 

 bably) owing to the order given by Ptolemy to his architect, Dinochares. 

 Ptolemy wished the roof of a temple at Alexandria to be roofed with the 

 magnet-stone, so that the own image of his sister, Arsinoe, should remain 

 suspended therein. But the death of the king and his architect prevented 

 the project from being carried out. 



The name "magnet" is said to have been derived from a shepherd named 

 MAGNES, who, when tending his flock on Mount Ida, found that his iron 

 crook was attracted to a certain stone ; and six hundred years before the 

 Christian era Thales wrote respecting amber and the magnet; and because 

 they attracted various substances, he supposed they possessed life and power. 

 They were the germs of the science now so developed in their applications, 

 and whose full powers we are scarcely yet acquainted with. 



We may remark that other bodies besides iron and steel are capable 

 of magnetization ; nickel and cobalt have the like property. Magnetism,, 

 properly so called, treats of certain bodies known as MAGNETS, describes 

 the properties they possess, and the influence of magnetic force upon other 

 substances. Electricity and magnetism are always associated, but practically 

 the force is the same, electricity being the current or motive power, so to 

 speak ; and it is to Faraday that the world is indebted for the discovery of 

 magneto-electricity. 



Epinus' theory of magnetism was that all bodies possessed a substance 

 he termed magnetic fluid, the partidcs of which repelled each other. But 



