SUPPLEMENTARY RULES. 



59 



the course of bathymetric curves several thousand meters below is a very striking 

 sign of this dependency. 



Sudden disturbances of the equilibrium will give rise to wave-motions. There 

 seems to be good evidence for the existence of large-scale waves in the bounding 

 surfaces between different layers in the sea.* Motions of the same kind are equally 

 possible in the atmosphere, and lines of flow of the character described in section 131 

 seem to show that they actually occur. When the motion has this character, we 

 have no right to expect a minimum of pressure along the lines of convergence and 

 a maximum of pressure along lines of divergence. In case of pure wave-motions, 

 the maxima of pressure should be under the summits and the minima under the 

 troughs of waves, while the line of convergence is under the front-slope and the line 

 of divergence under the back-slope of the advancing wave. If a progressive motion 

 is added, displacements of the lines of convergence and divergence take place, and 

 their relation to the pressure will not be easy to see on the chart. 



f f 136. Consequences of Kinetic Instability, Discontinuous Motions, and Eddies. 

 A kinetic phenomenon which is equally well known, though not so well understood 



vr//////////////777mM/. 

 C 



A. Eddy in a valley. 



Fig. 52. Motions due to kinetic instability. 



B. Eddies on windward and leeward side of a mountain. C. Eddy joining the great atmospheric motions. 



as that of the formation of waves, is that of the formation of eddies. They are 

 very often formed in the neighborhood of obstacles, both on the windward side of 

 the obstacles, and still more frequently behind them. A motion going on without 

 eddies is in such cases kinematically possible, but dynamically unstable, and has 

 therefore no chance of persisting even if it be produced for a moment. 



An eddy due to the instability of the motion may fill a valley across which there 

 passes a main wind (fig. 52 a). It may be produced both on the windward and on 

 the leeward side of a mountain (fig. 52 b), the latter case being the most frequent. 

 The observations of the wind at the ground will then give pictures like that of fig. 50 G, 

 with a parallel line of divergence and of convergence, the latter being as a rule the 

 one which appears most distinctly. The line of divergence may also disappear 

 completely when the eddy enters as a part of great atmospheric motions (fig. 52 c). 

 In such cases only a line of convergence will be discovered following the ridge of a 

 chain of mountains or the edge of a plateau-land. Eddies having a vertical axis 

 may be formed in the same way. This latter kind of eddy will be very frequent in 

 the atmosphere and perhaps still more so in the sea.f The eddies can exist on 



'Regarding this question on submarine waves, cf. Helland-Hansen and Nansen's work just quoted, Chapter VI. 

 See also V. W. Ekman's paper, "On Dead Water " (The Norwegian North Polar Expedition, 1893-96, Scientific Results, 

 Vol. V, Christiania, 1906). 



fConcerning eddies of large scale in the sea, cf. figs. 2, 37, 39, 105-107 of Helland-Hansen and Nansen's work just 

 quoted; and especially pp.311-312. 



