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 that the 

 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. 



