84 PHYSICAL GEOGRAPHY. The principal cause of constant ocean currents is the difference in the density of the equatorial and polar waters, produced by differences of temperature. The cold, dense waters of the polar regions tend to mix with the warm, light waters of the equatorial regions along due north-and-south lines. This tendency to north and south direction is prevented by the following causes: (1.) The rotation of the earth ; (2.) The position of the continents; (3.) The direction of the winds; (4.) The difference in the saltness of the water; (5.) The inequality of the evaporation and rainfall. In the Pacific, a counter-current crosses the ocean in the equatorial region, from west to east, In the Indian Ocean, the directions of the currents are modified by the land masses, which surround the northern part of its bed. In the northern hemispheres, the western borders of the oceans are colder than the eastern borders in the same lati- tude, because the former receive the polar currents and the latter the equatorial. Currents moderate the extremes of climate, by carry- ing the warm equatorial waters to the poles, and the cold polar waters to the equator. REVIEW QUESTIONS. ——0be300——_ How much heavier is salt water than fresh water ? What is the freezing-point of ocean water? | Explain the origin of the saltness of the oceanic waters. In the equatorial region, where is the water the colder, at the surface or near the bottom of the ocean ? How do the areas of the Pacific and Atlantic compare with each other in size? Of the Antarctic and Arctic? Define inland sea; border sea; gulf or bay; fiord; give examples of each. Define deeps; rises. What, most probably, is the shape of the bed of the At- lantic? Of the Pacific? Of the Indian Ocean ? Describe the Telegraphic Plateau. How does the greatest depth of the ocean compare with the greatest elevation of the land? Upon what does the height of a wave depend? On what does its velocity depend? What proof is there that during wave motion in deep water there is no continued onward motion of the water? Distinguish between ebb and flood tides. What proofs have we that tides are occasioned mainly by the attraction of the moon? What are spring tides? Neap tides? During what phases of the moon do they each occur? Why should the moon, which is so much smaller than the sun, exert a more powerful influence in producing tides? Where does the parent tidal wave originate? What are co-tidal lines? Why does the tidal wave progress from east to west? Explain the nature of the influence which the tidal wave exerts on the rotation of the earth. In what parts of the ocean will unusually high tides occur? Why? By what are races and whirlpools occasioned ? Distinguish between temporary, periodical, and constant oceanic currents. Explain the origin of constant currents. How are the directions of constant currents affected by the rotation of the earth and the shapes of the continents? What features of constant currents are common to each " of the three central oceans? On which side of the northern oceans do the polar cur- rents flow? On which side of the southern oceans? What are sargasso seas? How are they formed? What effect is produced by ocean currents on the ex- tremes of climate? Of what value are ocean currents to navigation? MAP QUESTIONS. ——-093.00——_ Point out, on the map of the river-systems, the inland seas of the Atlantic; of the Pacific; of the Indian Ocean. Point out the border seas of the Atlantic; of the Pacific. Point out the gulfs or bays of the Atlantic; of the In- dian Ocean. Point out the principal regions of fiords. How many hours does it take the tidal wave to progress from Tasmania to the Cape of Good Hope? From Tasma- nia to Newfoundland? From Tasmania to the British Isles? (See map of the co-tidal lines.) In what parts of the Atlantic does the tidal influence progress most rapidly ? If the velocity of any kind of wave motion in water in- creases with the depth of the basin, what parts of the At- lantic appear to be the deepest? What portions of the Pacific? What portions of the Indian Ocean? Trace on the map of the ocean currents, the motion of the Antarctic currents in each of the three central oceans. Where is the Cape Horn current? Is it hot or cold? What points ef resemblance exist between the north and south equatorial currents in the Atlantic and Pacific Oceans? 3 Trace the progress of the Gulf Stream. What points of resemblance exist between the Gulf Stream and the Japan current? How far to the north-east do the waters of the Gulf Stream extend? What distant shores are warmed by the waters of the Gulf Stream? By those of the Japan current? Why do not the heated waters of the Gulf Stream exert amore powerful influence on the climate of the eastern sea-board of the United States? Point out the, principal cold currents; the principal warm currents. Which currents would aid, and which would retard, the progress of a vessel in sailing from New York to San Fran- cisco? From America to Europe? -From America to India or Australia?