196 A NATURALIST IN THE PACIFIC ch. xin 



rising tide. During my second visit at the end of May, 1899, 

 when I was accompanied by Mr. Smallwood, I spent a night in 

 observing the behaviour of the spring tides at a spot below the 

 narrow portion of the passage 600 or 700 yards from the opening 

 on the coast. Here the breadth was about lOO feet, the depth at 

 low-water 5 feet, and the rise of the tide 4 feet. The current ran 

 seaward at a velocity varying from 1,500 to 2,500 yards per hour; 

 and it continued to flow in this direction for 2^ hours after the tide 

 had begun to rise on the coast. (In the narrow part of the 

 passage the rate of the current would probably be not over 3 

 knots.) It is curious that at the place of measurement the bottom 

 was formed of mud into which the pole sank six feet without 

 striking a hard substratum. The observations on the current were 

 made with a vertical float immersed about 3 feet. 



The point of difficulty in the behaviour of the tides is 

 this. The water is running rapidly out of the lake for nine hours ; 

 whilst during the remaining three hours there is a sluggish return- 

 flow up the passage into the lake. A far greater quantity of water 

 finds an exit by the passage than is returned by the same channel ; 

 and I can only explain this by assuming that there is an extensive 

 percolation of water from Natewa Bay into the lake. It is easy to 

 show that with such a narrow effluent, which cannot have a sec- 

 tional area exceeding 180 square feet, the level of the lake would 

 be only lowered 2 or 3 feet, if the average velocity during the nine 

 hours was two nautical miles. The great bulk of the water would 

 thus remain unchanged. The ultimate result of such conditions 

 would be a lake of brine. Since, however, the sea-water of the 

 lake possesses the ordinary density, it is apparent for this reason 

 only that there is some other means of supply than by the present 

 narrow passage leading to the sea. The mean level of the Salt 

 Lake is evidently rather above that of the sea, perhaps a foot 

 or two ; and the " rise-and-fall " is probably very small. 



