36 



We also need, as urgently, empiric tests on a much broader scale 

 than it has yet been possible to make, of the magnitude of the 

 error introduced into the calculations by the fact that even "the 

 deepest water-layer is actually not stationary, though it may be 

 moving so slowly that it is usually considered motionless for the 

 purpose of calculation. 



To check the magnitudes of these several sources of error, 

 regional dynaioic studies of the sea should be carried on hand in 

 hand with any direct means of discovering the velocity and the 

 direction of the current that may be feasible, whenever and wherever 

 opportunity allows. In the few cases where such a comparative 

 examination has yet been undertaken the agreement between the cal- 

 culated drift, and the type of circulation indicated by other lines 

 of evidence as prevailing at the time, has been close. Thus 

 repeated comparisons of the actual tracks followed by ice, drifting 

 down past the Grand Banks, has shown so good a correspondence to 

 the dynamic current charts made simultaneously by the Ice Patrol 

 cutters, as to warrant the hope that such calculations offer a 

 rational basis for predicting the drifts of individxial bergs 

 accurately enough to be of service to passing ships. 



Similarly, a recent dynamic study of the velocity and direction 

 of the outflow from the straits of Florida, based on observations 

 taken by the United States Geodetic Coast Steamer "Bache" in 1914, 

 agrees in general, with earlier measurements with current meters. 

 Dynamic circulatory tendencies calculated for different seasons of 

 the year for the Gulf of Maine are corroborated by various other 

 lines of evidence, direct as vjell as indirect; so, too, for the 

 Norwegian Sea, and for the northern sector of the Labrador current. 

 In short, we now have at hand a tool by which it is possible to 

 approximate, nui:ieriially, the movements of the whole mass of water 

 at a given time for situations where regional variations in specific 

 gravity indicate a drift much greater than the probable error, i.e., 

 where the current is certainly due to differences in hydrostatic 

 pressure. 



Other quantitative methods are needed, however, for situations 

 where the dynamic gradients are slight. Thus a method based on the 

 amount that the surface temperature departs from the value nornal 

 for the latitude and season, and on the thermal effects of evapora- 

 tion, recently worked out at the Scripps Institution for Oceanography, 

 and applied with promising results to the waters off the coast of 

 California, will probably prove generally applicable to other 

 regions where upwelling takes place on a broad scale. It also pro- 

 vides a useful check on horizontal velocities deduced from dynamical 

 causes. 



The perfection of quantitative methods, and the further aaipli- 

 fi cat ions of them that are to be expected, open tvra chief lines of 

 attack upon circulatory problems. In the first place, they set the 

 stage for a rapid advance in our knowledge of the stats of circulia- 

 tion actually prevailing over large ocean areas, and at all dapths 

 from the surface downward, especially for regions v/here there is a 

 wide regional variation in specific gravity. In fact recent dynamic 

 studies of the North Eastern Atlantic by Scandinavian oceano graph ers 

 have already materially altered the prevailing concept of the 



