230 DEPARTMENT OF THE NAVAL SERVICi: 



upwards, as indicated by the arrow in the figure. The highest portion of the layer 

 B is heavier than its surroundings, and has thus a tendency to downward movement 

 here similarly indicated. Thus we see, that according to the Archimedean principle, 

 the lower portion of the dividing surface will be raised, and the upper lowered; in 

 other words, the dividing surface will become horizontal. 



Fig. 7. — Archimedean forces in stable strata. 



-Movement occasioned by the forces 

 shown in Fig. 7. 



It is easy to understand what movements will be thus occasioned in the water 

 itself. The water will be forced from the thicker to the thinner portion of the 

 stratum, until linally a layer of uniform thickness is produced. We have thus 

 horizontal movements, proceeding in a direction from the thicker towards the 

 thinner portions of the layers, so that the forces indicated in fig. 7 occasion the move- 

 ments shown in fig. 8. 



Tills simple argument is, as we shall subsequently see, of fundamental import- 

 ance for the comTU-ehension of the actual conditions in the sea, and will also help 

 us to explain an interesting efiect of the wind. The svirface water in fig. 6 has been 

 transformed into a wedge-shaped stratum by the action of the wind. When the wind 

 drops, this layer will, according to the Archimedean principle, tend to resume its 

 normal luiiform thickness, the water being forced from the thicker towards the 

 thinner portion of the layer; i.e., in a direction contrary to that of the wind pre- 

 viously acting uix)n it. Thus the action of wind upon normally stable layers of water 

 occasions, upon its cessation, a movement in the surface layer contrary to the former 

 direction of the wind. 



It may also happen that water is continually introduced into some portion of 

 a stratum. The thickness of the stratum will then be greater at this point than in 

 the surroiniding portions, giving rise to Archimedean forces which drive the water 

 from the point of inflow horizontally towards the farther limits of the stratum. In 

 the tropics, great masses of water are heated by the rays of the sun. Such water 

 becomes specifically lighter, and passes over into the surface layer, which in the 

 tropics amounts to 600 metres depth, whereas at Spitzbergen it is only 200 metres. 

 Thus the Archimedean forces drive the water from the tropics towards Spitzbergen. 

 And the resulting current is that known as the Gulf Stream. 



From the foregoing, also, it will be iniderstood that the Gaspe current, the water 

 of which is formed in the mouth of the St. Lawrence river, becomes continually 

 shallower as it (i^roceeds. To the west of the Gaspe peninsula it must be deeper than 



