10 HiSTOKiCAL Introduction 



despite the fact that these terms are several orders of magnitude greater 

 than the terms of the driving force and dissipative force. William Ferrel 

 (1882) was one of the first to understand the role of the Coriolis force in the 

 distribution of ocean currents caused by the wind. He derived the relation 

 between the barometric gradient and the velocity of the wind (which is the 

 counterpart of the geostrophic current relationship of oceanography). For 

 some reason these remarkable achievements of Ferrel were overlooked by 

 oceanographers, and therefore his work did not have a direct influence on 

 the development of physical oceanography. The formula for computing 

 ocean currents from the slope of isobaric surfaces was derived by Henrik 

 Mohn (1885). 



The first careful study of the possibility of obtaining an approximation 

 to the velocity field in the ocean from an exact knowledge of the mass field 

 was made by Sandstrom and Helland-Hansen (1903), on the basis of 

 Vilhelm Bjerknes' (1898) circulation theorem. It became evident that the 

 CorioHs force acting upon the Gulf Stream is counterbalanced by a hori- 

 zontal pressure gradient associated "ndth the mass of fighter water in the 

 Sargasso Sea, and that the Stream is not so much a warm current as a 

 boundary phenomenon associated with, but not caused by, the sudden 

 downward slope of the isotherms toward the center of the ocean. It was 

 recognized that the differences in density across the Stream have nothing 

 to do with the driving of the Stream, but are simply part of an equihbrium 

 brought about indirectly by the stress of the wind. The great step forward 

 represented by this method of dynamic computation — as it came to be 

 called — was that at last oceanographers had begun to use the hydro- 

 dynamical equations of motion in their study of the sea, even though only 

 in a piecemeal way. 



By the turn of the century the dependence of density of sea water on its 

 temperature, salinity, and pressure had been determined. The ocean- wide 

 distribution of salinity had been charted and plotted, mostly as a result of 

 the pioneering work of Forchhammer (1865). This development came 

 surprisingly late in synoptic oceanography, especially when one considers 

 how much earlier the surface temperatures in most parts of the North 

 Atlantic had been known. 



THE TWENTIETH CENTURY 



We now enter the modern period of physical oceanography in which the 

 hydrodynamical equations of mean motion are used, although never in 

 their complete form. There are several reasons for using only a few of the 

 terms. First, the equations are nonlinear in their complete form, and 

 therefore the mathematical methods for solving them are imavailable. 



