QUESTIONS 169 



Waves are caused by up and down, not by lateral, move- 

 ment of the water affected. The power of waves and tides 

 to cause erosion results in their acting as destroyers of 

 unprotected shores. The solid matter eroded and carried in 

 suspension is often deposited at quieter places along shore. 

 Thus waves and tides may also act as builders. Ocean 

 currents are drifts of surface water, some of which, due to 

 the winds blowing over them, have very important effects 

 on the climate of adjoining lands. Tides are movements of 

 the water envelope of the earth caused by the rotation of 

 the earth and the gravitational attraction of the sun and the 

 moon chiefly the latter. Oceans furnish the water which 

 supports land life, food for thousands of people, and path- 

 ways of commerce for all nations. 



QUESTIONS 



What is the composition and what are the most striking charac- 

 teristics of water? 



Why does a freezing mixture freeze substances placed in it and 

 yet itself remain unfrozen? 



What is the difference between an emulsion and a solution? 



Why is soap used in cleaning? 



Explain the principle of the hydraulic press. 



How could a piece of lead be made to float in water? Why? 



Mention some ways in which ocean water differs from distilled 

 water. 



Waves and currents are both primarily due to winds. How 

 do they differ in action and effect? 



What are tides and their cause? 



Of what advantage is the ocean to man ? 



