1914] Sumner, et al.: Physical Conditions in San Francisco Bay 87 
respectively. The mean for the two bodies of water was about 31.7, 
a figure slightly higher than that for San Francisco Bay at the station 
nearest the Golden Gate, and about fifteen per cent higher than that 
for the bay as a whole. The absence of any large streams within the 
Woods Hole region, and the free intermingling of the coastal water 
with the ocean through the tides, are obviously responsible both for 
the higher mean salinity and for the small range of salinity in that 
region. 
In Chesapeake Bay, on the other hand, the salinity averages much 
lower than in San Francisco Bay. Figures are available from twenty- 
four stations within the upper two-thirds of Chesapeake Bay. These 
are derived from the ‘‘Manual of Oyster Culture in Maryland,’’ by 
Grave (1912, pp. 47-48). We have converted the density figures 
given in this report into corresponding ones for salinity, making due 
allowance for the temperatures at which the salinometer readings 
were made. 
The observations in Chesapeake Bay were made during two periods 
of the year: (1) during that of greatest rainfall (April 20 to May 8), 
and (2) during the period of least expected rainfall (October 10 to 
26).2° The mean salinity for all the stations for the first period was 
10.86, that for the second being 14.20. The mean of these two figures 
is 12.53. This represents an average salinity considerably less than 
half that of San Francisco Bay. It is a lower figure, indeed, than the 
annual mean for any of our hydrographic stations, even those in 
Carquinez Strait. 
The mean figure for the Chesapeake Bay station having the highest 
salinity (average for the two periods) was 18.90, that for the station 
having the lowest salinity being 4.33. 
The determination of water density by means of salinometers has 
been the one hitherto chiefly employed in this country and, despite 
the inexactness of the readings, this method will doubtless continue 
to be widely used on account of its greater convenience. We there- 
fore append table 23, for converting salinity figures of a wide range 
into those for specific gravity. In one column is given the specific 
gravity of various grades of salt water at 0° C, in the other their 
specific gravity at 15° C (= 59° EF). Most salinometers now used 
in this country are graduated so as to give correct readings at the 
latter temperature. All of the figures comprised in this table have 
23 As it happened, the rainfall for this latter period was much greater than 
usual, giving density figures which were doubtless lower than normal. 
