to mid-ocean. An important theoretical feature is the loca- 

 tion of the amphidromic (no-tide) points. Note that the 

 locations of these points are entirely different in Dietrich's 

 two charts, illustrating the frequency dependence of the 



response. 



In order to facilitate the preparation of more accurate 

 cotidal charts, a Working Group on Deep-Sea Tides was organ- 

 ized under the auspices of lUGG, SCOR and UNESCO, with Walter 

 Munk as chairman. This group is concerned with encouraging 

 tide observations in a grid of stations spanning the world's 

 oceans, improving the quality of deep-sea tide gauges, and 

 determining optimum analysis procedures for the measurements 

 that are made, usually at very large cost. 



Figure 11 shows the NOAA tide gauge being lowered into 

 the water. The gauge was subsequently modified by replacing 

 time releases that were found to be not dependable by acous- 

 tic release mechanisms. Figure 12 shows the same gauge after 

 it has left its tripod base on the sea floor and returned to 

 the surface. The two gauges, presently at a depth of about 

 4,000 meters on the Caribbean sea floor, have additional 

 floation above the instrument frame to offset the added weight 

 of the acoustic releases, a larger frame and a current meter. 



A numerical experiment on spacecraft tides is described 

 in the following article, reproduced by permission of the 

 American Geophysical Union. The paper, by Bernard D. Zetler 



26-18 



