288 



FIRST YEAR SCIENCE 



the sun and moon upon its water envelop. The moon is 

 much the more effective because it is nearer. 



In the open sea the rise and the fall of the tides are of 

 equal duration but at the head of bays the tide rises more 

 rapidly than it falls so that low tide does not occur mid- 

 way between two successive high tides. 



136. Corals and Coral Islands. In warm, clear, shallow 

 areas of the sea are found small animals called corals. 



PANAMA 



An example of man's domination over nature. 



Greai colonies of these are able by united action to 

 build barriers capable of withstanding the waves and of 

 raising submarine lands into islands. These reef -building 

 corals cannot live at a greater depth than 20 fathoms (120 

 feet), or where the mean temperature is lower than 68 F., 

 or where the discharge of rivers makes the water-fresh or 

 muddy. As they are fixed animals and must get their 

 food from the surrounding water, they flourish best where 



