4 
in depth. The sea above the abyssal water, furthermore (with the 
exception of comparatively restricted places, such as a turbulent 
mixing zone during a gale), is in a condition pronouncedly stable. 
Winter cooling of the surface layers, it is true, sets up temporary, 
vertical, convectional currents, but this condition is short lived 
when we consider the entire year’s span. 
DYNAMIC CONSIDERATION OF A WATER MASS 
In support of what has just been remarked, we might continue by 
regarding a vertical section of a stable water mass devoid of 
circulation. We will find the densest water rests on the bottom of 
the basin; the lightest water on the surface; and the isosteric surfaces 
will be exactly horizontal. If now a water particle from a bottom 
layer be shifted to the surface it will begin to sink to the isosteric 
sheet from which it was removed. A surface particle, just as truly, 
if submerged to the bottom will tend to rise and return to its former 
level. But if a sample be taken from one position to another posi- 
tion, all within the same layer, then there is no force giving rise to 
its return. It is obvious from this that water particles resist any 
tendency toward removal from their own particular isosteric sheet, 
but may move freely within such, if friction does not hinder the 
motion. 
Every motion may be regarded simply as a displacement of masses, 
therefore a study of various types of distribution of mass in the sea 
is bound to reveal a vast deal regarding the currents, and in this 
respect the extreme importance of isosteric bounds governing the 
movements of the water particles can not be over emphasized. It 
will be seen, therefore, in the light of further remarks that once we 
have determined the general contour of the isosteric surfaces we have 
gained an insight, not only of the direction in which the water is 
moving, but also a measure of its relative rate of flow. The well- 
known principle of Archimedes is of great assistance in clarifying 
the components of the forces due to varying densities. 
Let-us again regard in profile a vertical section of any body of sea 
water wherein a distribution of density prevails from which dynamic 
variations may easily follow. Such a case may arise, as we have 
pointed out, as an effect of either one of two classes of forces. (See 
internal and external forces, page 1.) For example, imagine thatthe 
ocean has absorbed and mixed heat unevenly during the summer, 
causing the water to become lighter in a zone over a shallow coastal 
shelf than the water farther offshore; or perhaps an abnormal per- 
centage of onshore winds have amassed a quantity of light water 
from the surface layers against a coast. Here, then, class (1) or 
class (2) forces have produced similar results which can best be ex- 
amined by recourse to a vertical section normal to the coastal trend. 
