424 



WELLS'S NATURAL PHILOSOPHY. 



Such a needle, when properly balanced, will be 

 observed to vibrato more or less, until it settles in 

 such a direction that one of its extremities, or 

 poles, points toward the north, and the other to- 

 ward the south. If the position of the needle bo 

 altered or reversed, it will always turn and vibrato 

 again until its poles have attained the same direc- 

 tion as before. 



It is this remarkable property of a magnetized 

 steel bar, of always assuming a definite direction, 



that renders the compass of such value to the mariner, the engineer, and th» 



traveler. 



The ordinary compass consists of a mag- 

 netic needle, or bar balanced upon a pivot, and 

 inclosed within a shallow box, or metallic case. Upon 

 the bottom of the box is a circular card with the chief, or 

 cardinul points of the horizon, north, south, east, west, 

 marked upon it. 



Fig. 354 represents the form and construction of the ordinary, or land com- 

 pass. The term compass is derived from the card, which compasses, or in- 

 volves, as it were, the whole plane of the horizon. 



What ia 

 Compass 7 



What is the 

 construction of 

 the Sea, or 

 Mariner's Com- 

 pass? 



In the Sea, or Mariner's Compass, the needle is attached to 



the under side of the card, in such a way that both traverse 



together — the needle itself being out of sight. Upon the 



surface of the card is engraved a radiating diagram, dividmg 



the whole circle of the horizon into thirty-two parts, called 



points. The compass-box is supported by means of two concentric hoops, 



called gimbals. These are so placed as to cross each other, and support the 



box immediately in the center of the two ; so that whichever way the vessel 



