250 



H-l 



vectors. Since the directions and velocities of the current are 

 repeated, with some variation, at intervals of the periods of the tidal 

 cycles, and since high and low water at any tidal station in the same 

 region are repeated at nearly the same intervals, the times marked on 



the diagram generally are re- 

 '-■*"3 ferred to the times of high and 



low waters, or of the principal 

 current phases, at a well-estab- 

 lished tidal station. 



488. Shapes of polar current 

 curves. — In regions where the 

 tides are of the semidiurnal type 

 the currents are nearly identi- 

 cally repeated during each suc- 

 cessive semidiurnal tidal cycle, 

 and the current curve usually 

 has an elliptical shape, exempli- 

 fied by the mean current curve 

 at Nantucket Shoals Lightship, 

 figure 82, taken from the Man- 

 ual of Current Observations, 

 United States Coast and Geodetic Survey (Special Publication No. 

 215). The times marked on the diagram are referred to the times of 

 high and low water at Boston. Thus "H— 2" marks the current 2 

 mean solar hours before high water at Boston, and "L+3" the current 

 3 hours after low water at Boston. 



489. In regions where the diurnal mequality of the tides is con- 



H+3 



I Knots 



J 



Figure 82.— Mean current curve for Nantucket Shoals 

 Lightship, referred to tides at Boston. ■ 



LLtl 



LL+2 



LL+3 



Figure 83.— Tidal Current Curve, Swiftsure Bank Lightship. Eefer-red to predicted time of tide at 



Astoria, Oreg. 



siderable, the currents during the two semidiurnal cycles have a 

 corresponding inequality, and the daily tide curve describes a double 

 loop, exemplified by the mean current curve at Swiftsure Bank Light- 



