THE OCEAN 341 



that it had the day before. This makes the period between suc- 

 cessive high tides 12 hours and 26 minutes. 



The movements of the tides are not so simple as the outline above 

 would imply. Many things interfere. The continents stop the 

 advance of the tidal waves, and they travel more slowly in shallow 

 than in deep water. Since tides are retarded most near continents 

 and islands, their advance is here most irregular. 





 



Fig. 267. Diagram to show why it takes nearly 25 hours for a given place to 

 come twice into the same relation to the moon. The earth rotates in 24 

 hours, and at the end of that period, a point, as a, has made the circuit. 

 But the moon, which was at M t at the beginning, has advanced to M 2 , so 

 that a must move on to a' , before it has the same relation to the moon 

 that it had the day before. 



Solar tides. The sun also attracts the earth, and tends to 

 cause tides. If there were no moon we should still have tides pro- 

 duced by the sun. Though the sun is very much farther from the 

 earth (about 93,000,000 miles) than the moon is; yet because of 

 its great size it attracts the earth much more strongly than the 

 moon does. But the tides produced by the sun are less than half 

 as high as those produced by the moon. The reason is that the dif- 

 ference between the attraction of the sun (1) for the center, and (2) for 

 the side of the earth nearest it or farthest from it, is much less 

 than the difference between the attractions of the moon for the 

 same points. The tides which are actually felt on the earth are 

 the result of the influence of the moon anfl the sun; but since the 

 moon's tides are much the stronger, the sun's tides merely modify 

 them. The sun strengthens the tides when sun and moon work 

 together, and weakens them when they work against each other. 



Spring tides and neap tides. When the sun and the moon stand 

 in the relation to each other and to the earth shown in Fig. 268 



