24 University of Califorma Publications in Zoology [ Vou. 14 
As judged by the foregoing table, the mean tidal range throughout 
San Francisco Bay, for the month as a whole, is 4.52 feet (1.38 meters). 
This range varies from 3.49 feet (mean of the neap tides) to 5.46 feet 
(mean of the spring tides). These figures are based upon the semi- 
daily range, i.e., the difference of level between succeeding high and 
low tides. The spring ranges represent the greatest semidaily ranges 
for the month, the neap ranges the least semi-daily ranges for the 
month. 
The figures above given are, however, averages, based upon numer- 
ous measurements. For a given station, the actual difference between 
the greatest and the smallest tide occurring in the course of a year 
would be very much greater than this table would indicate. Thus 
at Fort Point, in the Golden Gate, ranges as low as 0.4 foot and as 
high as 7.8 feet occur. Furthermore, it must be borne in mind that 
these are predicted rather than observed ranges, and that they take 
no account of temporary conditions, such as the state of the wind or 
the rate of discharge of the rivers. Thus the actual extremes may be 
even greater than those just given. 
A computation, based upon twenty-four entire days, taken at equal 
intervals, throughout the year 1912,?° gives us the mean length of the 
period between low and high water as 6 hours 19 minutes, and the 
mean length of the interval between high and low water as 6 hours 
7 minutes. It must be stated, however, that on any given day no such 
approximate equality is commonly maintained between the period of 
rising and of falling tide. Either phase may be in excess. Indeed, 
one phase may last more than twice as long as the other. 
These periods must not be confused with the periods of the ‘‘flood’’ 
and ‘‘ebb’’ currents, respectively. As is well known, the time of high 
or of low water may not correspond at all to the time at which the 
direction of the tidal current is reversed. We read in the United States 
**Coast Pilot’’? (1909) that ‘‘the ebb current runs out much longer 
than the flood runs in through the entrance to San Francisco Bay’’ 
(p. 74). 
It has already been shown that during the mean ebb tide about 
1,077,600 acre-feet of water pass out of San Francisco Bay, or about 
17.7 per cent of the volume present at the time of high water.*t This 
10 This computation is based upon the figures given in the ‘‘Tide Tables.’’ 
The twenty-four hour period commencing with the first change of tide on the 
first and fifteenth day of each month was taken. 
11JIn this computation it has been assumed that the various parts of the bay 
reach high water mark simultaneously. This is not quite true, since high tide 
