1914] Sumner, et al.: Physical Conditions in San Francisco Bay 71 
flood, 26.88; ebb, 27.57. Moreover, these relations hold both for sur- 
face and for bottom. The figures for these considered separately are: 
Flood Ebb 
f aa SSS 
Surface Bottom Surface Bottom 
26.00 27.75 26.94 28.21 
Again, if we exclude the four stations (see footnote on p. 64) 
for which we cannot fairly compare the flood and ebb observations, 
we find that at fifteen of the remaining nineteen stations the flood 
figure is the lower. 
1 | SS SS SS Se 
Feb. Mor. Apr. May Jun. vu. Aug. Sept Oct. Nov. Dec. Jon. 
Fig. P. Annual salinity curves for entire bay. The continuous line is 
based on flood figures, the broken line on ebb ones. 
Since the mean salinity of the bay increases with tolerable regu- 
larity from Carquinez Strait to the Golden Gate, it might have been 
expected that the flood stream would, in general, shift water from a 
zone of higher salinity to a zone of lower salinity. The explanation 
of this unexpected condition is, however, quite simple. As in 
another case already referred to, the anomaly doubtless depends upon 
the procedure followed in making our observations. 
If, at any given point, water samples were taken continuously 
through a succession of tidal phases, it would be found that, in general, 
the salinity would begin to rise with the onset of the flood stream, 
reaching its maximum at about the period of slack water. With the 
commencement of the ebb, the salinity would gradually fall again 
until it once more reached its minimum. Early flood observations 
would thus show practically the same salinity as late ebb observa- 
tions, and an even lower salinity than early ebb observations. The 
higher salinity shown by our ‘‘ebb’’ figures is quite intelligible on 
the assumption that the observations were made preponderatingly 
during the earlier half of each tidal phase. 
