CHAPTER IV. 



CURRENTS. 



Origin of ocean currents Evaporation and precipitation Water-circulation in 

 Mediterranean and similar seas Effect of wind on currents Temporary 

 currents caused by wind Currents running counter to prevailing winds Tidal 

 currents " races " or "roosts "Velocity of littoral currents increased by 

 breakwaters projected across them Effect of currents upon height of waves 

 Strong currents out of harbours objectionable. 



IT is believed that the heat of the sun's rays and the rotation of 

 the earth upon its axis whether acting directly upon the waters 

 of the ocean or through the medium of the winds are the two 

 principal forces which produce and give direction to ocean 

 currents. These must not, however, be taken to include tidal 

 currents, which owe their origin to the tidal wave being impeded 

 in its progress, as will be more fully described later on (p. 69). 



Whatever causes an inequality in the level of the surface 

 of the ocean, or in the density or specific gravity of its waters, 

 disturbs equilibrium, and a current, which may be regarded as 

 an effort of nature to restore the lost balance, is at once set up. 

 The subject of ocean currents is, however, as yet far from being 

 perfectly understood. 



Water, in common with other substances, expands with 

 heat, and in so doing its specific gravity is diminished. If, 

 therefore, any portion of the ocean be heated more than the 

 water which surrounds it, its bulk will be increased, and, as a 

 natural consequence, its surface will be raised. This will cause 

 the warm-water to flow off, in order to find its normal level ; 

 but the warm- water column, being thus relieved of weight, will 

 continue to rise, on account of the lower and heavier stratum of 

 cold water forcing itself underneath it. This cold water will, in 



