2 
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 forees 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 wecan 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 d=den- 
sity, and v=specific volume, then v=7° As an example of the con- 
tractions which are customarily adopted by practical hydrographers, 
we may have given, d=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. 
