SOME CHARACTERISTICS OF SEA WATER STRUCTURE 649 



because it represents the actual structure of the sea as observed. It has 

 many advantages in the study of regime and mechanism in oceano- 

 graphic systems, because it emphasizes many phenomena which are 

 readily overlooked in a natural-plot. 



Linear Interpolation of Data 



When data from serial observations are plotted on a natural depth 

 scale, the gradients are usually interpreted by a free-hand continuous 

 curve as shown in Figure 1. In many cases this is subject to dispute, 

 and in any case it does not define the limits of the zones. Furthermore, 

 the structure in the zones near the surface is lost in the small depth 

 scale, which is necessary to include the deep zones where very little 

 structure is evident. 



The log-plot provides a conventional interpretation of serial oceano- 

 graphic data which is realistic, and acceptable. The linear interpola- 

 tion between the elements of data, and the unmistakable definition oi 

 the zones are ideals which have long been sought. Furthermore, the 

 display of the data coincides with the degree of interest in most oceano- 

 graphic studies, because it emphasizes the structure in the zones near 

 the surface, while retaining the definition of the deep zones, and the 

 continuity of the whole structure. 



Demarkation of Zones 



In the log-plot the limits of each zone are sharply defined by the 

 intersections of the straight lines. This characteristic allows a more 

 critical examination of the data than any other technique of interpreta 

 tion. Furthermore, it is not necessar}' to have continuously recorded 

 data to determine the limits of a zone; it is only necessary to have 

 enough observations to define the gradients of the properties. 



The intersection of the gradients at the limits of the zones suggests 

 that the structure is discontinuous at these points. There is some doubt 

 whether these limits are real discontinuities or not. There is some 

 lag inherent in the S.T.D. and bathythermograph, which are the best 

 lecording instruments available, so that the slight rounding of the cor- 

 ners shown in their records are not altogether reliable. Most records 

 show a small step at the limits of the zone, such as illustrated in Figure 

 5, but the significance of this has not been determined. 



Index Salinity 



In the examination of these data it has been noted that the salinity 

 at the upper limit of the more homogeneous zones and sub-zones tends 

 to remain constant along the line of flow. For this reason, it is termed 

 the index salinity. However, the salinity at the upper limit of the 

 boundary zone is not constant. On examination of the data in Georgia 



