122 DYNAMIC METEOROLOGY AND HYDROGRAPHY. 



Passing to the charts of pressure differences, we get this perfectly analogous 

 result: The charts for pressure differences between successive level surfaces show 

 the course and the number of equipotential-isopycnic unit-tubes in the sheet 

 between two levels. 



On the charts of mutual topography (figs. 14 and 20) the curves are drawn for 

 differences of height of 10 dynamic meters, i. e., for 100 dynamic decimeters. Thus 

 between the vertical walls represented by the curves there run 100 isobaric-isosteric 

 unit-tubes. On the charts of pressure differences (figs. 16 and 23) the curves are 

 drawn for the intervals of pressure of 1 m-bar, i. e., 0.1 c-bar. Between the vertical 

 walls represented by these curves there will consequently run 0.1 equipotential- 

 isopycnic unit-tubes, if the m. t. s. units be used. 



74. Complete Representation of the Fields of Moving Forces and Moved 

 Masses in the Atmosphere. Our aim has been to arrive at a complete represen- 

 tation of the fields of pressure and of mass. But it is worth while mentioning that 

 in reality we have attained more than this. 



For the investigation of atmospheric equilibrium and motion a third field, that 

 of gravitational force, is of fundamental importance. Being invariable, this field 

 need not, like the changing fields of pressure and mass, be specially represented. 

 But it merits attention that in our representations of the variable field of pressure 

 is implied also that of the invariable gravitational field. 



The charts giving the dynamic topography of the isobaric surfaces are repre- 

 sentations of the gravitational field of force tangentially to these surfaces. Men- 

 tioning the charts of dynamic topography of the earth's surface and of the bottom 

 of the sea, we have already developed the idea of these charts as representing 

 two-dimensional fields of force (section 18). Evidently a combination of the two- 

 dimensional fields for the succession of isobaric surfaces will give a complete 

 representation of the three-dimensional field in space. 



The other representation of the field of pressure is by isobaric curves drawn on 

 level surfaces. Now, the level or equipotential surfaces give themselves a direct 

 representation of the gravity potential and thus of the gravitational field of force. 

 It is the field of pressure, which is represented in the more indirect way, as the 

 field of gravity potential in the preceding case. We have here a perfect parallelism. 

 The isobaric charts in level surfaces represent the two-dimensional fields of the 

 pressure gradient in these surfaces, just as the topographic charts of the isobaric 

 surfaces represented the two-dimensional fields of the potential gradients in these 

 surfaces. The comprehension of these isobaric charts for the successive levels give 

 the representation of the three-dimensional field of the pressure gradient in space. 



Whichever of the two methods we choose, we thus get simultaneously a repre- 

 sentation of the fields of force due to gravity and to pressure. At the same time, 

 the charts of relative topography or of relative pressure represent the field of mass. 

 We have thus obtained a complete representation of the fields both of the moving 

 forces and of the masses moved. 



