tion, that due primarily to land and bottom configuration as it tends 

 to guide and shape the direction as well as to effect the velocity of 

 ocean currents. Friction also is an important factor, arising whenever 

 water particles of dissimilar motions interact among one another. 

 A well-known example of this process is contained in the waters of a 

 mixing zone which lies adjacently inshore of the Gulf Stream and 

 stretches along the American continental slope. 



It is difficult, even in such a well-known current as the Gulf 

 Stream, to state which class of forces, internal or external, is the 

 fundamental cause of movement, yet the subsequent forces tending 

 toward alterations of the movements spring from two influences — 

 friction and rotation of the earth. A discussion of some of the fore- 

 going features will assist to a clearer understanding of the entire 

 subject. 



STATIC CONSIDERATION OF A WATER MASS 



Let us imagine that we can pass a plane vertically downwards through 

 the ocean and can regard a cross section of the water in profile, with a 

 view to studying its static condition, or distribution of mass. If now 

 the water particles could be colored with reference to their relative 

 weights, we would find the lightest water in the surface layers, and the 

 heaviest particles on the bottom. The two fundamental essentials 

 usually determined and which lead to hydrostatic examination are tem- 

 perature and salinity; once they are found the specific gravity (density) 

 follows as a dependent from convenient hydrographical tables. It is 

 often desirable to speak in terms of specific volume, it being the volume 

 of a body per unit mass, or the reciprocal of the density. If ^ == den- 

 sity, and 1; = specific volume, then v = -j' As an example of the con- 

 tractions which are customarily adopted by practical hydrographers, 

 we may have given, cZ= 1.02711; this is written, for the sake of 

 brevity, 27.11. The corresponding value of v in this case is 0.97361, 

 and this is often shortened to a numeral of only three digits, 

 viz, 361. The greater the specific volume at any point the lighter 

 the water is there. 



If now we return to our vertical section in the sea and connect all 

 points wherein the water particles have the same specific volume for 

 differences of every 10 units of the latter, we obtain a number of lines 

 called isosteres running throughout the profile. An isoster is a line 

 all points along which represent like values of specific volume; an 

 isosteric surface merely increases the consideration to the two dimen- 

 sions of an area. An isosteric surface may be visualized as spread 

 out beneath the surface of the sea— an undulating floor whose depth 

 can be determined with the same reality as the more tangible floor 

 of the ocean is sounded out by the hydrographer. 



