4) 
In Figure 2 the oblique lines are isosteres which have been formed 
by the intersection of the vertical plane of the section with the 
isosteric surfaces running through the water mass. The space 
between any two isosteric surfaces is called an isosteric sheet. The 
uppermost isosteric sheet on the left-hand side of Figure 2, in wedge- 
shaped form, bounds the body of lightest water that has accumu- 
lated against the coast. Now the water in the deepest portion of this 
isosteric sheet ‘‘A” is specifically lighter than the water at the same 
level in any of the other isosteric sheets, so according to the Archi- 
median principle this portion of sheet “A” will tend to be driven 
bodily upwards. The water in the highest portion of sheet “B” is 
specifically heavier than the water at the same level of the inshore 
sheet, and thus it will be dragged downwards. It is plain to see that 
pp /OC* 
Fic. 2.—A vertical profile of a water mass showing a distribution of light and heavy 
water and dynamic tendencies which would prevail in such a state 
there are forces tending to turn all the isosteric sheets into a hori- 
zontal position, and the greater the obliquity of the isosteric surfaces 
the greater the forces tending toward the leveling process. The 
water particles themselves, however, as a result of these stresses, 
will be forced from the thicker portion of the isosteric sheet to the 
thinner portion of it, the particles tending to keep, for reasons as 
pointed out in a previous paragraph, wholly within their own re- 
spective layer. When the sheets have attained a mean uniform thick- 
ness, then the isosteric surfaces have resumed the horizontal, dynamic 
equilibrium is established and circulation ceases. 
The causes provoking currents were divided, it will be recalled, as 
being due to two classes of forces—yviz, internal and external. The 
distinction between the two rests mainly on the manner in which 
energy is transmitted to the sea. This conception should be clearly 
understood. 
(1) Internal class of forces refers to those agencies, the effects from 
which appear forthwith to alter the internal character of the water 
mass itself. This results in varying the distribution of density. 
