258 



the subsequent subtraction of the constant component current, is 

 facihtated by adding to each component velocity an arbitrary con- 

 stant sufficiently large to make all of the quantities positive. The 

 direction and velocity of the resultant constant current may be 

 obtained from its components, after the subtraction of any arbitrary 

 constant that may have been added for the convenience of computa- 

 tion. The algebraic subtraction of the constant component of the 

 velocity from the hourly current components in either direction, gives 

 the hourly components of the tidal velocity in that direction. The 

 curve of the average tidal velocities proper may then be constructed 

 by finding the resultant hourly tidal currents. If the period of 

 observation is less than a month, the tidal velocities may be reduced 

 to better mean values by multiplying them by the ratio of the estab- 

 lished mean tidal range at the reference station to the average range 

 during the period in wliich the current observations were made. 



507. Wind currents. — Analyses of the current observations at light- 

 ships have afforded useful information on the strength and directions 

 of the currents produced by winds in open waters. The results 

 indicate that as a general rule, along the Atlantic coast, the velocity, 

 in knots, of the current, produced by a wind of some duration, is 

 about 1 K percent of the wind velocity in miles per hour ; and along the 

 Pacific coast, about 2 percent. Because of the rotation of the earth, 

 the direction of the current tends to lie to the right of the direction of 

 the wind in the Northern Hemisphere, and to the left in the southern, 

 A Swedish mathematician, V. W. Ekman, has shown that if the depth 

 of the ocean was unlimited, the surface wind currents would have a 

 direction 45° to the right of the wind in the Northern Hemisphere, 

 and 45° to the left in the southern. (Arkiv for Mathematik, Astro- 

 nomic, 1905). A comparison between the recorded deviation of 

 vessels from their courses and the direction and strength of the winds 

 causing the currents to which the deviations may be attributed, is 

 said to confirm these relative directions of wind and current (Marmer, 

 The Tide, p. 165). Near the coasts, the direction of the current with 

 respect to the wind is modified by the configuration of the coast line. 

 Thus the current observations at the light vessels from San Francisco 

 to Cape Flattery show that the winds from the northeast, southeast, 

 and northwest quadrants produce currents which set 20° to the right 

 of the wind direction, winds from the southwest quadrant produce 

 currents 20° to the left, and winds from the south and west produce 

 currents which set with the wind. 



It need not be remarked that these offshore currents are of more 

 concern to the navigator than to the engineer. 



