251 



LL+3 



LH-2 



N 



LL+2, 



LL+ 



Figure 81.— Tidal Current Curve, San Francisco I,ight3liip. Referred to predicted time of tide at 

 San Francisco (Golden Gate), Calif. 



ship, off the entrance to the Strait of Juan de Fuca, figure 83, taken 

 from the same source. 



The times of the currents on this diagram are referred to higher liigh 

 water (HH), lower low water (LL), lower high water (LH) and 

 higher low water (HL) at Astoria, Oreg. 



490. At some tidal stations the second current loop may become 

 ^ery small, as shown in the mean current curve at the San Francisco 

 Lightsliip, 10 miles off the entrance to San Francisco Bay (fig. 84). 



The current swings through a nearly complete circle, and then 

 swings backward and forward through a limited arc before it resumes 

 its swing around the compass. The behavior of the currents at this 

 and other similar stations varies greatly with the declination of the 

 moon. At the time of equatorial tides the curve has two nearly 

 equal loops and the current swings around the compass twice during 

 the day. At the time of tropic tides, the secondary loop becomes 

 very small or vanishes altogether, and the current makes but one 

 daily swing entirely around the compass. 



491. Combination of constant and rotary currents. — In figure 85, 

 PP1P2 is the current curve, and 0, at its geometrical center, is the 

 pole, of a rotary tidal current. The vector OP then represents the 

 direction and velocity of the tidal current at a given time. 



192750—40 17 



