276 Our Surroundings 



with tanks inside, into which sea water may be admitted by 

 valves. If enough water is added, the vessel goes below the sur- 

 face. Since, however, water pressure increases rapidly with 

 depth, a submarine cannot descend too far without danger of 

 crushing. When the water is pumped from its tanks the sub- 

 marine rises. 



A modern submarine is crowded with intricate machinery. 

 There are, for example, oxygen tanks and apparatus to purify 

 air, gauges to indicate depth below the surface, pumps and valves 

 to regulate the depth, oil engines to propel the ship on the surface, 

 electric motors and storage batteries to drive it when submerged, 

 and appliances to handle weapons if used in war. 



Stability of Ships. The safety and value of a ship depend 

 largely on its design and on its stability. Stability is capacity to 

 stay upright, or to recover an upright position instead of rolling 

 over. In general, stability is increased by having all possible 

 weight in the boat placed low and near the center. There are 

 five main factors in making boats stable : having wide beam, or 

 great width ; building air chambers into the sides ; weighting the 

 keel, the lowest timbers or plates in the ship's bottom; placing 

 ballast, or heavy material, low in the center of the vessel's hull ; 

 using a centerboard, a thin, weighted blade extending below the 

 keel. Sailboats are built with centerboards or weighted keels 

 to overcome the tendency to overturn, due to the wind pressure 

 on the sails. 



Ocean Routes. Gradually from long experience, steamships 

 have come to follow regular paths or lanes. These lanes are 

 many, many miles in width. They represent the courses which 

 give greatest freedom from storm, fog, and danger of icebergs. 

 These traffic lanes are not the same at all seasons of the year. 

 For example, in crossing the Atlantic, ships take a more northerly 

 course in summer than in winter. 



Most governments have charted their coast lines, marking all 

 rocks, reefs and shallow areas so that sea captains may be guided 

 in keeping their ships in safe waters. In addition, governments 

 establish and maintain lighthouses at points of special danger, 

 and buoys, or painted posts or floats, to mark the channels of 



