Transportation on Water 375 



it from the center to one end with one pole of the magnet, and 

 then from the center to the other end with the other pole. It 

 may be done without direct contact. Try it both ways. When 

 done by contact the slight jarring hastens the molecular changes 

 in the steel. The action of the earth is much slower, because 

 its field is comparatively weaker than that of the magnet. 



In magnetizing the iron bar or pipe, hammering accelerates 

 the change of the molecules into alignment with the magnetic 

 field of the earth. The suspended bar responds much more 

 slowly. It is just the same with steel and iron ships, and steel 

 structures on land. They all become magnetized in time by 

 the earth. 



The theory illustrated. The way in which the molecules of an iron 

 bar arrange themselves in response to the earth's magnetic influence 

 may be imagined by representing the molecules as little magnets which 

 are parallel and have their north-seeking poles at one end and the 

 south-seeking poles at the other. In contrast with this arrangement 

 of the molecules in a magnetized bar, we may represent their ar- 

 rangement in a non-magnetized bar as so irregular that they have no 

 combined influence so far as this can be detected by the magnetic 

 needle. 



166. The gyro-compass. As stated above, ships of steel 

 and iron interfere seriously with the use of the magnetic com- 

 pass in navigation. A new instrument has been invented and 

 perfected in late years, which has proved to be so dependable 

 and satisfactory that it is now in use in practically all the navies 

 of the world as well as on the modern ocean liners. This in- 

 strument is the gyro-compass, and is so named because it makes 

 use of the gyroscope as the principal mechanism. 



The gyroscope, which should become familiar to every one, 

 is one of the most interesting mechanical devices. It is virtually 

 a top. The heavy wheel with its axle in one piece with itself 

 is mounted in a ring on pivots (Fig. 134). When the wheel is 

 made to rotate at high speed by means of a string or otherwise, 

 it resists very strongly any external force that would change 



