Transportation on Water 



371 



164. The lodestone. The earliest compass was probably 

 made of lodestone, which is widely distributed in nature. The an- 

 cient Greeks were familiar with it and some of their scientists de- 

 scribed its property of attracting and repelling iron particles. 

 The ore is a compound of iron and oxygen, known as magnetite 

 (page 434). Lodestone has the capacity of imparting to iron 

 or steel its magnetic properties with no loss to itself, and also 

 the property directly associated with the compass, of assuming, 

 if left free to move as when floating on a bit of cork, a definite 

 direction with respect to the .earth, that is, pointing nearly 

 north and south. 



165. A simple compass. It was early learned that a bar 

 or iron or steel, when magnetized, behaved like the lodestone, 

 pointing nearly north and south. A 



steel needle was therefore substituted 

 for lodestone in the compass, an im- 

 provement greatly increasing its use- 

 fulness. The needle was then, as now, 

 balanced in a horizontal position on 

 a pivot (Fig. 132). The important 

 fact is not the way in which it is 

 pivoted, but the magnetic property 

 of the needle itself, because it may 

 be suspended by a long hair or a 

 single fiber of silk, or floated on a bit 

 of cork on water. Such a simple compass is useful for experi- 

 mental study of the behavior of the needle. 



The ordinary compass is made of a magnetized bar of steel 

 supported horizontally on a steel or jewel pivot above a gradu- 

 ated disk marked N., S., E., W., and inclosed in a brass case 

 covered with glass. The mariner's compass (Fig. 133) is much 

 more complicated, but exactly the same in principle. The circle 

 on the disk is divided into degrees and then into thirty-two parts, 

 each 11^. The instrument is mounted so as to maintain a 

 horizontal position whatever may be the position of the vessel. 



FIG. 132. A simple compass. 



