180 PACIFIC SCIENCE, Vol. I, July, 1947 
STAGES 
ALTERNATING MOVEMENTS OF MEAN LINE 
0 100 
COMPOSITION DIAGRAMS 
TIME- 
Fig. 4. Schematic diagram showing widening of the transition zone with successive cycles of alter¬ 
nate invasion of one type of water by the other. The lower part shows five composition diagrams, 
commencing with the sharp boundary at the left. The general sigmoid form of the composition line 
may be verified by plotting the figures given in the right-hand column of Figure 3. 
differences through larger openings in dif¬ 
ferent zones, we cannot doubt that there 
will be slow and persistent growth in the 
width of the zone because of this rinsing 
effect. Thus we suppose that, setting any 
given limits at the two margins, the thick¬ 
ness of the transition zone will become 
progressively greater as alternations are re¬ 
peated, or as the movement assumes greater 
amplitude. 
The composition diagram developed on 
this assumption of rinsing is a symmetrical 
curve starting as an asymptote to the zero 
line of salinity, passing through an inflexion 
at the 50 per cent point, and terminating as 
an asymptote to the line of 100 per cent 
salinity of sea water. After any series of 
complete cycles the zone has widened, but 
the line of equivalent density or balance is 
found in the position formerly occupied by 
the initial sharp boundary. 
REVERSIBILITY 
We seem now able to resolve the often 
discussed problem of reversibility of the 
process of saline encroachment. This has 
vexed many people in Hawaii and seems to 
be clarified by the above discussion. From 
that line of reasoning, we may say at once 
that if we consider the position of the 50 
per cent line, or the equivalent position of 
total salt against total fresh water, the pro¬ 
cess of saline encroachment does appear to 
be completely reversible. After a period of 
reversed movement equal in duration and 
intensity to the earlier saline encroachment, 
assuming each movement completed, we 
