TIDES 



287 



The tidal currents are also continually changing the 

 water in bays and harbors and thus keeping them from 

 becoming stagnant and foul. They also bring food to 

 many forms of shore life which have but little or no power 

 of movement, such as clams and other shellfish, and by 

 exposing some of these at the ebb give man a chance to 

 acquire them readily for food. 



THE VATERLAND. 



One of the largest vessels afloat. 



It has been found that the position of the sun, as well as 

 that of the moon, affects the height of the tide. If the 

 earth, moon and sun lie in nearly the same line, the tidal 

 range is greatest. This is called spring tide. When the 

 sun and moon act at right angles upon the earth, the tidal 

 range is least and this is called neap tide. The tidal undu- 

 lations have been proved by astronomers to be due to the 

 rotation of the earth and the gravitational attraction of 



