934 BULLETIN OF THE BUREAU OF FISHEHIES 



hence, it must be taken into account in some way in calculating the dynamic slope 

 at the surface. 



Jacobsen and Jensen (1926) have very fully discussed this question in their 

 dynamic study of the Faroe Channel, finding that in most cases this effect of the 

 bottom water may be sufficiently allowed for by arbitrarily applying to the dynamic 

 gradient between the two stations in question the product of the difference in 

 specific volume between them at the deepest level of the shoaler station multiplied 

 by half the difference in depth. If the station where the calculation shows the 

 surface as highest also has the largest specific volume at the deepest level of the 

 shoaler of the pair, the gradient is to be increased by the amount of this correction — 

 decreased if the reverse obtains. If the difference in depth be greater than, say, 150 

 meters or so, no arbitrary correction of this sort can be relied upon, consequently 

 the dynamic gradient can be stated only within very wide limits. The only cure is 

 to establish the stations closer together on future cruises. 



The dynamic-contour chart °° closely resembles an ordinary weather map in its 

 general appearance, and it is as easily interpreted in terms of the resultant circula- 

 tion. Dynamically, the water tends to flow down the slopes from the parts of the 

 picture where the surface stands high to those where it is low, and at right angles to 

 the contour fines. Actually, however, this could happen only at the equator. Every- 

 where else the effect of the earth's rotation so deflects this motion that the stream 

 lines come nearly to parallel the contour lines, which may then be taken as directly 

 representing the current, just as the direction of the wind is roughly parallel to the 

 isobars on the weather map. 



In the open ocean, where tidal currents are weak, the contour lines may even 

 approximate the tracts of the particles of water if approximately constant accelera- 

 tion has been estabfished. This, however, does not apply in a region such as the Gulf 

 of Maine, where the tidal currents average much stronger than the dynamic tendencies. 

 In this case the latter act only to give to the tidal flow a character more definitely 

 rotary than would otherwise be the case, or to strengthen the one tide at the expense 

 of the other. Here the dynamic-contour lines show only the general advance which 

 the water tends to make good in its tidal oscillations to and fro. 



Because in every case the datum plane for the calculation is necessarily the 

 underlying water, not the solid bottom of the sea, the motion indicated by the chart 

 is not absolute, but is only relative to that of the deepest stratum of water included 

 in the picture. If this be motionless, the calculated drift represents the actual 

 motion of the surface (or chosen level) relative to the coast line, but not otherwise. 



In the Northern Hemisphere, where moving bodies are deflected to the right, 

 the direction of flow, relative to the plane of reference," is to be identified by the 

 rule that the gradient current will constantly have the lightest water (i. e., the 

 highest surface) on its right hand, the lowest surface on its left, as it veers cyclon- 

 ically around the latter. If the surface drift be faster than the bottom drift, as is 

 usually the case, this indicated direction of flow will also be the true direction, 

 relative to the bottom; so, too, if bottom and surface drifts be parallel, whichever 



•0 Dynamic-contour charts may as easily be constructed for any desired depth below the surface of the sea, as described by 

 Smith (1926). 



" In the Gulf of Maine this is the bottom water between the pairs of adjacent stations. 



