10 MANUAL OF CURRENT OBSERVATIONS 
LL+3 
LH-2 
LH-! 
LL+2e 
LH 
LH+t 
is ae 
= of HH-2 
\ Zan LHt+e 
S55 
WSN 
HL-2 
NV 
HL-1 
North LL 
eee 2S 
LUI oe 
0.5 knot 
FIGURE 12.—Mean current curve for San Francisco Lightship, referred to tides at San Francisco. 
Swiftsure Bank Lightship which is located off the entrance to the Strait of Juan de Fuca. 
In this graph the radii vectores are referred to the times of the tides at Astoria, Oregon, 
separate references being made to higher high water (HH), lower low water (LL), lower 
high water (LH), and higher low water (HZ). The graph shows two loops covering 
the tidal day and illustrates the diurnal inequality in current velocity. 
23. Figure 12 represents the mean current at San Francisco Lightship which is 
located about 10 miles offshore from the entrance to San Francisco Bay. In this graph 
the secondary loop is comparatively small and indicates a change of direction of the 
current through a limited are rather than around the entire circle. The current is 
ereatly modified by changes in the declination of the moon and for the time of the 
equatorial tides the graph would show the two loops nearly equal in size with the current 
swinging entirely around the compass twice during the day. At the time of the tropic 
tides, the secondary loop would become very small or vanish altogether so that the 
current becomes practically diurnal and requires all day to swing entirely around the 
compass. 
24. When a nontidal current is combined with a rotary tidal current, both the 
velocity and direction of the latter are affected. This is graphically illustrated by 
