35 



effect of the wind on the other. Until mathematical expressions 

 were made available to take account of all the factors (e.g., wind 

 and internal friction, regional differences in specific gravity, 

 deflective force of earth rotation) qtiantitative measurement of the 

 velocity of currents, in the sea, could be made only by current 

 meter. But, generally speaking, the use of these instruments is 

 confined to sl.oal waters near land, i.e., to situations where tidal 

 currents are not only strongest, but are veering if not reversing, 

 hence where they so constantly confuse the picture that continuous 

 observations over long periods are necessary before the dynamic or 

 other broad scale movements can be distinguished from the local 

 and temporary ones. 



Many such current measurements have been taken on special tidal 

 surveys along the various coastlines, likewise from light shios in 

 the North Sea, in the Baltic, and off the east and west coasts of 

 North America; also in the straits of Florida, where Pillsbury 

 carried out his classic studies of the volime and velocity of the 

 outflow from the Gulf of Mexico. But, by the nature of the case, 

 quantitative estimation of the drift of the whole mass of water for 

 any considerable area of the open ocean demands more generally 

 applicable and deductive methods. And for the development of these 

 we must thank the theoretic advances in ocean physics that have 

 followed Bjorknes' development of hydro-dynamics, chiefly at the 

 hands of Scandinavian oceanographers; likewise Eckman's mathematical 

 discussions of the proble.a of wind currents. 



_ Mathematical calculation of dynamic circulation has now been so 

 simplified that the method can be mastered by any phvsical oceano- 

 grapner. 5\irthermore, the raw data that are needed are not only 

 easily obtained but are of a sort that have long been collected in 

 ordinary routine, i.e., a record of the teiaperature and of the 

 salinity at a sufficient number of depths-levels, and at a net of 

 stations sufficiently close, and taken nearly enough simultaneously, 

 to allow Horizontal projection of the dynamic state prevailino- over 

 the area as a whole. Unfortunately, however, neither this method 

 j_nor, for that matter, any other indirect method) can be the cure-all 

 xnat Its simplicity and its mathematic defensibility might suo-^est 

 because of certain vory serious sources of possible error. In'the' 

 iirst place the results are only relative to some other mass of v;ater 

 which may be taken as the base for calculation. Consequently, unless 

 hP vlni'^''^*^^ °^ the water chosen as base be measured, or unless it 

 be know., to oe stationary, the calculated result cannot give the 

 t^r^i °^??'^?^ /owever, in favorable cases record of temperature 

 and of salinity for a sufficiently dense network of stations can be 

 so dealt with as materially to overcome this difficulty. Secondly, 

 the contour of the bottom introduces a factor that can seldom be 

 stated nu:-aerically, for if the dynamic current strike a ridge of the 

 ^n?-ffi^SJ'/'' a coastline, it may be given a character quite 

 different fror.; that calculated for the "free ocean," of which all 

 oceanograpners speak so glibly, but which no one of us will ever see. 



rs^o.^ol^r^'^^'^^i ""'^ urgently need some general expression of the 

 ?h! Jf t? U-'i? ^""^^ calculations are applicable to regions where 

 the depth differs much from station to station, or to condensate 



Stio^^i S? S""^ ^??^"" °^ ^^P*^' ^°^^ numerical allowance more 

 rational than the arbitrary corrections that have so far been proposed. 



