132 Js'atural History of the Ocean^ ^-c. 



depth ; but it is estimated to extend only fifteen fathoms 

 below the surface. According to Boyle's experiments, the 

 wind never afFects the water lower than six feet; so that 

 in a calm, the waves of the ocean never exceed twelve 



#rH?<k 



feet. 



In equatorial regions, the waters must be more agitated 

 than in the polar regions, from the centrifugal force. When 

 the water is very deep, waves have no progressive motion, 

 but remain In situ : but if the bottom is rocky, and not very 

 deep, the waves are interrupted, sent back, and form break- 

 ers. 



Oil spread on the surface of the sea, smooths the w^ater, 

 in some measure, as was noticed by Dr. Franklin; and 

 among the ancients by Aristotle, Pliny, and Plutarch, who 

 mentions that the divers took oil in their mouths, and let it 

 out when under the water, to smooth tlie surface above 

 them. In the Bermudas, oil is now used to render the wa- 

 ter clear for fishing. 



Dr. Forster at sea in 1797 saw that the grease thrown 

 overboard by the Cook affected the water to a great dis- 



tance ; and throw^ing overboard a quantity of oil, he found 



that it calmed all that portion of the waters. He adds, a 

 tcaspoonful spread over several yards : and explains the 

 effect by saying, that the wind could not get a purchase to 

 raise the first rimples, which by increasing, become waves. 

 II. Motion of Currents. m - 



J ■ 



1. Equinoctial currents. Between the Tropics therein 

 a constant motion of the sea from east to west, at the rate 

 of nine or t^n miles in the twenty-four hours, which seems 

 caused by the trade winds. Commencing on the West 

 coast of America, and running with great violence to the 

 east coast of Africa, thence on Asia, and again back to the 

 east coast of America. Its course is determined by the 

 land it meets with, and, accommodating itself to the coast, 

 seems to have assisted in forming the Bay of Bengal: whence 

 it runs to the Coromandel coast between Ceylon and Asia. 



2. Polar currents, running to the Equator, are mention- 

 ed by Humboldt, and Scoresby, with a velocity of from one 



to three miles an hour. Their temperature is lower than 

 the surrounding ocean. 



3. Upper and under currents; or counter currents, ap- 

 pear in gulphs, bays and enclosed seas. The Atlantic flows 

 into the Mediterranean superficially, while the surplus wa» 



