200 CORAL AND ATOLLS 



travel from tlie south of the lagoon to the north — a distance 

 of about eight miles. The amount of tidal variation is an 

 important factor in determining island elevation, and this will 

 be again referred to in the consideration of the mode of 

 formation of the islands. 



The lagoon being shallow, calm, and more or less enclosed, 

 is considerably warmer than the surrounding ocean, and some 

 parts— as for instance the back water enclosed by the sand 

 spit at the northern end of Pulu Panjang— are distinctly hot 

 after a day of calm and sunshine. A surface temperature 

 of 85° Fahr. is a very common one for the open waters of the 

 lagoon. 



