234 



DEPARTMEXT OF THE NATAL SERVICE 



repeated several times, until equilibrium is attained. When this occurs, the isosteres 

 will be in preeist^^their old position, and the state of the water exactly what it was 

 before the commencement of the wind. 



The sea-water-, thus reacts more after the manner of an elastic body than a fluid, 

 when subjected to the influence of forces acting upon it. The forces in question can 

 only occasion a certain degree of deformation, and as soon as they cease to operate, 

 the water returns to its former condition. External forces produce, so to speak, effects 

 as upon a mass of jelly. 



The Archimedean forces thus play very much the same part in sea-water as that 

 of elasticity in a solid. It may occasionally happen that the external forces acting 

 upon the sea water reach a magnitude exceeding the highest possible value which can 

 be attained by the Archimedean forces. A catastrophe then takes place, and an 

 entirely new state of things is brought about, exactly as when the limit of elasticity 

 in a solid is exceeded. 



The maximal value of the Archimedean forces is reached when the isosteres become 

 vertical. Should external forces exceeding this maximal value be brought into play, 

 then the water layers will jibe over, as shown in fig. 12, which illustrates the distribu- 

 tion of density under a very strong wind. Now, however, light water is brought down 

 beneath heavier; the stability is upset, and a strong vertical convection ensues, whereby 

 the whole of the water affected is mixed up into one single homogeneous layer. The 

 specific volume of this layer will, of course, be equal to the mean specific volume of 

 the strata of which it was composed, and between this and the water beneath there will 

 be a sharply defined difference of density. In the homogeneous layer, there are no 

 isosteres, and consequently, no Archimedean forces will be formed, so that the water 

 therein contained will follow without resistance the direction of external forces acting 

 upon it. The circulation here will therefore be highly intensive, as sh<twn in fig. 13, 

 so that the water will continue homogeneous, and exert a continual friction upon the 

 layer beneath, thus maintaining the sharply defined limit of density between the two. 

 This friction occasions a gradual absorption of the water from the lower layer, so that 

 the upper one w'ill tend to increase in volume and specific gravity. 



Fig. 12. — Water layers jil»ing in a high wind. 



Fig. 13. 



-Strong circulation in surface 

 layer after jibing. 



When the external forces which brought about the transformation have sub- 

 sided, and the isosteres have resumed their horizontal position, the distribution of 

 density will naturally not be the same as before the forces in question had commenced 

 to operate, vide fig. 10. Instead of this, we now find a homogeneous surface layer of 

 great volume, and between this and the water beneath, a sharply marked break in the 

 density, vide fig. 14. This is the reason why the greatest variation of density with 



