[DAWSON] EFFECT OF WIND ON CURRENTS AND TIDAL STREAMS 181 



surface as a wind drift, and oniy when long continued will its influence 

 extend to any great depth. 



In limiting our discussion, therefore, to the question of the disturb- 

 ance of the current, it may be broadly stated that the tendency of the 

 current to flow in the same direction as the wind, is due to the combined 

 influence of the wind itself and to difference of barometric pressure over 

 wide areas. In the case of the Gulf of St. Lawrence, when the pressure 

 is exceptionally high or low over so large an area, the effect should be all 

 the more noticeable; as the corresponding flow has to take place through 

 comparatively narrow entrances or straits. 



Amount of the disturbance. — When the writer first began these in- 

 vestigations, the general impression derived from books v,^as that the 

 current would always be found to set in the same direction as the wind. 

 But the longer the investigations were carried on, and- -the greater the care 

 to assign each movement of the water to its true cause, the less residuum 

 there remained to ascribe to the vv^ind, as otherwise unaccounted for. 



There appear to be several reasons which may largely account for 

 this impression that the current goes with the wind : (1) A faulty method 

 of observation, by which the drift of small floating objects was taken to 

 represent the set of the current. It is vrell known that the wind will set 

 a film or skin of smooth water in motion in a few minutes ; but to accept 

 this as the direction of the current is specially reprehensible on the part 

 of nautical men, for whom the surface current should mean its move- 

 ment at a depth of at least half the draught of an ordinary vessel ; as its 

 speed at this depth represents its average effect on a vessel. (2) The 

 difficulty of distinguishing leeway from current drift, especially in the 

 old sailing-ship days. Possibly even yet there are few masters of vessels 

 who have ascertained accurately the exact leewa}^ made by their vessels 

 for each given force of the wind. This was carefully done for the first 

 vessel used on this Survey ; but even then the residual current drift de- 

 ducible was uncertain. (3) The great complexity of the tidal stretims 

 in the regions under consideration. A strait may connect two regions 

 in which the tide is governed by distinctly different astronomical condi- 

 tions; and before this was ascertained it was assumed without due con- 

 sideration that the observed complexity of the tidal streams was due to 

 the wind. (4) It is noteworthy that in obtaining information from 

 fishermen, only the least observant men speak in a vague way of the 

 current running with the wind. The more intelligent men attribute less 

 to the direct action of the wind, and distinguish the various effects more 

 carefully. 



Methods and appliances. — The general method employed in the in- 

 vestigation of the currents, is to anchor the surveying steamer at care- 

 Sec. III., 1909. 11. 



