LESSONS IN GEOLOGY. 335 



easterly winds from the Indian Ocean furnish abundance of mois- 

 ture, but during summer the central region becomes so hot 

 and the air so light that exceptional winds from the Atlantic 

 ocean flow into that portion of Africa. These winds both leave 

 abundant moisture on the slopes of the plateau, but the greater 

 part falls after the upward current is formed , condensed by cold 

 from elevation. 



Thus the winds carry the water from the sea to the land, where 

 a portion becomes permanent in the rocks and soil, a portion 

 evaporates and a portion forms springs, brooks and rivers. 

 These inland waters are very efficient geological agents. 



The sea, with waves, tides and currents, is also an important 

 force in geological work. 



Waves are caused by the wind and are common on the sea every- 

 where varying in elevation with the force of the wind. The wave 

 has a forward movement, but the water only moves upward and 

 downward. When waves advancing toward the shore reach shal- 

 low water, the motion is retarded at the bottom by friction, and 

 the top moving on without support breaks over on the shore, thus 

 forming breakers. During storms waves break on the shore with 

 great force. 



Tides are waves caused by the attraction of the sun and moon. 

 There are four tidal waves, two of equal size on opposite sides of 

 the earth caused by the moon, and two much smaller caused by 

 the sun. As the moon revolves around the earth once in about 

 27 days, the waves caused by the moon make the circuit of the 

 earth in the same time. Twice during this circuit the moon waves 

 coincide with the sun waves, making higher or spring tides, and 

 twice the lower part of one coincides with the higher part of the 

 other, and a lower or neap tide is formed. As the earth rotates 

 in the same direction that the moon revolves, a given place 

 passes through the crest of both moon waves once in 24 hours 

 and 52 minutes. In mid ocean the tide does not rise more than 

 three feet, but along some coasts and in narrow bays they rise 

 much higher. At the head of the Persian Gulf the tide rises 

 36 feet; at the Bay of St. Michael, France, 45 feet. In shoal 



