72 University of California Publications in Zoology [ Vou. 14 
Now, a scrutiny of our original records shows that this assumption 
is quite justifiable. It was in general our aim to commence our obser- 
vations for the day at the very beginning of one or the other phase 
of the tide. In about a third of the cases it is expressly recorded 
that this was done, while the same was doubtless true in many other 
instances. Our field notes likewise show that the hydrographic obser- 
vations for a given day extended through an average period of less 
than three and three-fourths hours, and consequently must commonly 
have been completed long before the tide turned. In those few cases 
in which it was necessary to-trespass on the ebb period, in order to 
”? 
complete the ‘‘flood’’ observations for the day, or vice versa, the 
figures have been rejected from our computations. 
lood Observations 
} Eb Observations 
i 
| 
x 
=) 
iS 
3 
5} 
6 
Fig. Q. Diagram illustrating explanation of higher mean salinity of water 
samples taken during ebb tide than during flood tide. 
The diagram figure Q will illustrate the foregoing explanation 
of the average higher salinity of our “‘ebb’’ water samples. The 
curves here figured are, however, considerably generalized. The 
actual conditions shown throughout any single tidal cycle are far from 
being as simple as this, as will be seen by a glance at figure R. It is 
plain, first of all, that the surface and bottom waters have changed 
more or less independently of one another. If we may judge from 
the salinity, the turn of the tide for the former occured at about 
10 a.m., which corresponds to the time of high water, as stated in the 
“Tide Tables.’’ On the other hand, the fall of salinity for the bottom 
water did not occur until some two hours later. This was likewise 
the time at which the ship began to swing about. The next time of 
low water, according to the ‘‘Tide Tables,’’ was 3:17 p.m., which cor- 
responds neither to the time of reversal of the current, as observed 
on the ship, nor to the rise of salinity at the surface or bottom. 
