228 



DEPARTMENT OF THE NATAl. SERVICE 



3. DYNAMIC IMPOKTANCE OF THE ISOSTEEIC SURFACES. 



The stable position of the strata in these Avaters is in many respects characteristic 

 of the movement occasioned therein. In order to make this clear, we may take a simple 

 example. Fig-. 4, let us say, represents a sea basin filled with homogeneous water, i.e. 

 in which no isosteric surfaces occur, subjected at the surface to the action of wind 

 blowing in the direction of the larger arrow. The water will then commence to circu- 

 late in the manner indicated by the small arrows, the current thus induced increas- 

 ing continually as long as the wind lasts. Take, then, fig. 5 as representing a basin 

 containing different water layers in stable equilibrium, for the sake of convenience, 

 we may presume that only three water layers of different specific gravity are repre- 

 sented. The layers themselves will then all be devoid of isosteric surfaces, but in the 

 dividing surfaces between the layers, a large number of isosteres will be found. When 

 not subjected to the action of external forces, the three different kinds of water will 



Wind 



Fig. 4. — Wind currents ni homogeneous watei'. 



arrange themselves in horizontal strata, the specific gravity increasing with the depth. 

 Should, now, a wind arise, it will act upon the surface of the uppermost and lightest 

 layer, the water of which this is composed being forced along in the direction of the 

 wind. This layer will then become wedge-shaped, as shown in fig. 6, its water at the 

 same time circulating in the manner indicated in fig. 4. Owing to the friction thus 

 caused, a certain amount of the movement in this surface layer will be communicated 

 to the layer immediately beneath, which in its turn begins to circulate, but in the 

 opposite direction, the combined movement exactly corresponding to that of two cog- 

 wheels working together. Finally, the bottom layer will be similarly set in motion by 

 the one above it, and will commence to circulate in the same direction as the surface 

 layer, albeit at a slower rate. 



Fig. 5. —Water layers in stable juxtaposition. 



Wind 



Fig. 6. — Wind cun-ents in stable water layers. 



