SKCT. XXX. THE MARINER'S COMPASS. 305 



But it is said that a rude form of the compass was in- 

 vented in Upper Asia, and conveyed thence by the 

 Tartars to China, where the Jesuit missionaries found 

 traces of this instrument having been employed as a 

 guide to land travelers in very remote antiquity. From 

 that the compass spread over the East, and was imported 

 into Europe by the Crusaders, and its construction im- 

 proved by an artist of Amalfi, on the coast of Calabria. 

 It seems that the Chinese only employed twenty-four 

 cardinal divisions, which the Germans increased to 

 thirty-two, and gave the points the names which they 

 still bear. 



The variation of the compass was 'unknown until Co- 

 lumbus, during his first voyage, observed that the needle 

 declined from t^ie meridian as he advanced across the 

 Atlantic. The dip of the. magnetic needle was first no- 

 ticed by Robert Norman, in the year 1576. 



Very delicate experiments have shown that all bodies 

 are more or less susceptible of magnetism. Many of 

 the gems give signs of it ; cobalt and nickel always pos- 

 sess the properties of attraction and repulsion. But the 

 magnetic agency is most powerfully developed in iron, 

 and in that particular ore of iron called the loadstone, 

 which consists of the protoxide and the peroxide of iron, 

 together with small portions of silica and alumina. A 

 metal is often susceptible of magnetism if it only contains 

 the 130,000th part of its weight of iron, a quantity too 

 small to be detected by any chemical test. 



The bodies in question are naturally magnetic, but 

 that property may be imparted by a variety of methods, 

 as by friction with magnetic bodies, or juxtaposition to 

 them ; but none is more simple than percussion. A bar 

 of hard steel, held in the direction of the dip, will be- 

 come a magnet on receiving a few smart blows with a 

 hammer on its upper extremity ; and M. Hansteen has 

 ascertained that every substance has magnetic poles 

 when held in that position, whatever the materials may 

 be of which it is composed. 



One of the most distinguishing marks of magnetism is 



polarity, or the property a magnet possesses, when freely 



suspended, of spontaneously pointing nearly north and 



south, and always returning to that position wiien dis- 



20 c c 2 



