858 BULLETIN OF THE BUREAU OF FISHERIES 



Readings of the current meter (or the simpler method of employing a float) 

 give the rate of the current over a known interval of time and its du-ection.'^ These, 

 then, are reduced to average velocities and directions for each tidal hour after the 

 time of high water at some neighboring station of reference, and it is in this form 

 that they appear in the current tables published in the United States Coast PUot 

 (United States Coast and Geodetic Survey, 1911, p. 151) and in the current tables 

 for the Bay of Fundy (Dawson, 1908). In all such tables the direction stated is 

 that toward which the current flows, referred to the true meridian. In other words 

 a "northeast" current is just the opposite of a "northeast" wind. 



To plot the course which an imaginary body, floating in the water, would travel 

 during the period from one high tide to the next, is perhaps the most graphic way 

 to bring out the existence or absence of a dominant drift at any given locality. If 

 the flood and ebb currents are exactly opposite in rate, duration, and direction, 

 such a float would return precisely to its starting point, for there would be no 

 resultant drift. In all probability, however, this would never happen in any part of 

 the Gulf of Maine. If, with ebb and flood opposite in direction throughout their 

 respective duration, one were stronger than the other, a dominant set would result 

 parallel to the direction of the stronger. This condition is to be expected in nar- 

 row channels, such as the Grand Manan Channel, and close in along some parts of 

 the coast line; but in most parts of the gulf the direction of the tidal current 

 changes from hour to hour, running in a comparatively constant direction for only a 

 few hours when ebb or flood is at its strength. In some localities the tidal current 

 is perfectly rotary, with its direction veering uniformly throughout the half-tidal day. 

 Such a state, for example, is to be expected about 16 miles to the eastward of Nan- 

 tucket Shoals light vessel (United States Coast and Geodetic Survey, 1912, p. 10). 



In the Gulf of Maine and on its offshore banks tidal currents veer always to 

 the right — i. e., with the hands of the clock — most rapidly, in most cases, at the 

 times of high and low water. Thus, a particle of water or any floating object, such 

 as a buoyant fish egg, drifting during a tidal period, would follow a course varying 

 in different parts of the gulf from a closed circle (bringing it back close to its 

 starting point), through various types of veering spirals, to courses nearly opposite 

 in direction for the two tides but unequal in distance. In most parts of the gulf, 

 therefore, any such floating object would not follow the dominant or nontidal set 

 directly, but in a zigzag or spiral course, traveling a much greater distance in the 

 daily tidal components than the distance made good along the azimuth of the non- 

 tidal set. 



The dominant set that results from a veering current may be deduced in various 

 ways. If calculation be preferred, an approximation is easy with the ordinary navi- 

 gational traverse tables in precisely the same way the navigator calculates, from 

 his dead reckoning, the distance and course made good for the day. 



In most cases a graphic method of summation is to be preferred. The following 

 (now in common use and recently described in detail by Mavor (1922)) is, perhaps, 



" It should be borne in mind that in tabular statements of currents the words " velocity " and "distance " are not synony- 

 mous; for, obviously, if the current is flowing at a rate of 1 mile per hour at one hour, and at 2 miles per hour an hour later, the 

 distance made good during the interval is neither 1 mile nor 2 miles, but the mean of the two. This caution is added because 

 some of the published tables of currents have been ambiguous in this respect. 



