SECT. 5] 



INTERNAL, WAVES 



735 



The isotherm followers have been employed singly, or in triangular arrange- 

 ments of three, to acquire the. speed and direction of internal waves by phase 

 shifts (LaFond, 1959). Continuous operations up to periods of five weeks have 

 been successfully conducted (Fig. 4). 



Fig. 4. Thi'ee isotherm followers in operation from booms suspended out from the U.S. 

 Navy Electronics Laboratory oceanographic tower. 



3. Observed Relations 



Internal waves affect vitally all elements whose nature is influenced by 

 temperature or density changes in the water. Such factors include sea life, the 

 chemical and physical properties of water (including currents and acoustic 

 penetration), and the superficial structure of the ocean bottoms. 



A. Characteristics in Shallow Water 



Internal waves have been measured in all oceans and in several lakes through- 

 out the world. Off southern California, ^ in summer, the vertical oscillation of 

 the temperature structure was measured by use of thermistor beads suspended 

 vertically at a depth of 50 ft (LaFond, 1959b), and later by isotherm followers 

 at a 60-ft depth (Lee and LaFond, in press). In this series of experiments, 

 the following characteristics of internal waves were determined. 



1 Mission Beach, San Diego. 



