CHAPTER VI. 



SUPPLEMENTARY RULES TO ASSIST IN THE DRAWING OF THE LINES OF 

 FLOW AND OF THE CURVES OF EQUAL INTENSITY. 



133. Remarks on the Digression. We have emphasized the fact that the 

 drawing of the lines of flow and of the curves of equal intensity would cause no 

 difficulty, if we had at our disposal a sufficient number of really good observations ; 

 but as a matter of fact the observations are often so scarce and so heterogeneous 

 that great doubts arise as to the course of the lines. In such cases we must look for 

 other diagnostic methods than the pure kinematic ones. 



This leads us to give here, in anticipation, diagnostic rules depending upon 

 dynamic, partly also upon thermodynamic and other principles. The foundation 

 of these rules will be considered more fully in later parts of this work. Deviating 

 thus for practical reasons from the strictly theoretical plan, it will be important to 

 make certain reservations in connection with this digression. 



If the aim be simply this, to find the most probable motion of atmosphere 

 or hydrosphere on a certain occasion, it is perfectly legitimate to bring into applica- 

 tion all diagnostic methods which may serve the purpose. But if further con- 

 clusions should be drawn from the picture of motions thus obtained, we must take 

 care to avoid the circulus vitiosus. If rules derived from dynamic or thermodynamic 

 principles have been used to produce the picture of atmospheric motions, this 

 picture can not be used legitimately afterwards to verify these same rules. 



It can not therefore be too strongly recommended to develop the system of 

 direct observations of atmospheric and hydrospheric motions, in order to make it 

 possible to arrive at the synoptical representations of the motions by methods of a 

 purely kinematic nature. Representations obtained in this way will be the only 

 ones which can be legitimately used for subsequent investigations regarding the 

 dynamic and thermodynamic phenomena which are the causes of the motions. 



134. Relation of the Kinematic Singularities to Dynamic and Thermodynamic 

 Phenomena. Motion has a general tendency to go from higher toward lower pres- 

 sures. From this we easily derive the following special rule : 



Within a barometric depression there is a probability for existence of points 

 or lines of convergence; within areas of high pressure there is a probability for the 

 existence of points or lines of divergence. Long ridges of high pressure will as a 

 rule contain a line of divergence; long ridges of low pressure a line of convergence. 

 In the neighborhood of a maximum-minimum point of pressure situated between 

 two high and two low areas there will be a probability for the existence of a neutral 

 point with hyperbolic lines of flow. 



Where the given observations of the wind do not give sufficient evidence for the 

 nature and placement of the singularities, the required supplementary evidence 

 may be obtained by examining the chart of pressure. But in doing so we should 

 remember that there is no necessity for the motion to go always, and under all con- 



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