90 THE OCEAN. 



the space of a lunar day, that is to say, within the 24 hours 50 

 minutes during which the earth has successively presented all parts 

 of its surface to the planet which accompanies it, the two waves 

 ought each to accomplish a complete circuit around the globe, and 



Fig. 27.— Syzygy tide, during the new Moon. 



each should have a total duration of 12 hours 25 minutes. This is, 

 in fact, what takes place over all seas. As to the numerous vari- 

 ations presented by this phenomenon, in its height and the precise 



