94 DYNAMIC METEOROLOGY AND HYDROGRAPHY. 



of the Appendix is used. The table shows that the level curve of the height zero 

 coincides with the isobaric curve of a pressure of 750 mm. mercury at sea-level, 

 independently of the temperature. The level curve of a height of 50 dynamic 

 meters coincides almost completely with the isobaric curve for a pressure of 755 

 mm. mercury, deviating for high temperatures towards the isobaric curve of 754 

 mm. mercury, and for low towards the isobaric curve of 756 mm. mercury. In the 

 same way the curve of 100 dynamic meters of height closely follows the isobaric 

 curve of 760 mm. mercury, with small deviations towards higher pressure for low 

 temperature and towards lower pressure for higher temperature, and so on. Using 

 this table and the isothermic chart, slight changes are easily made in the isobaric 

 curves, giving thus the level curves representing the topography of the 1000 m-bars 

 surface. 



Table 19 A of the Appendix serves the same purpose, in the case of the isobaric 

 chart being drawn for inches of mercury and the isothermic for Fahrenheit degrees. 

 This table has been used to draw the topographic chart for the 1000 m-bars sur- 

 face in fig. 13 from the corresponding isobaric chart for sea-level published by the 

 U. S. Weather Bureau. 



The principle for the calculation of tables of this kind is explained in the next 

 article, where tables serving an analogous purpose are described. 



From the charts of absolute topography, obtained by extrapolation from below, 

 those of relative topography, representing the distribution of mass in the sheets 

 between the standard surfaces, may be deduced at once by the method of arithmetical 

 or graphic subtraction. The arithmetical method will generally be found preferable 

 on account of the acuteness of the angles of intersection of the curves of absolute 

 topograph}* (section 63). Two charts obtained in this way are given in fig. 20. 



67. Construction for Lower Levels of Charts of Absolute Pressure and of 

 Pressure Differences from Observations at the Earth's Surface. Drawing the 

 extrapolated virtual-temperature diagram as explained in section 58, and calculating 

 the pressure in standard levels, we can draw the isobaric charts of absolute pressure 

 in these levels. Afterwards, by the method of arithmetical or graphic subtraction 

 (the first being generally preferable), the charts of relative pressure, representing 

 the distribution of density in the level sheets, can be drawn. 



On the other hand, if the charts of absolute topography of standard isobaric 

 surfaces be drawn, it is easy to change them into isobaric charts for corresponding 

 standard levels. To see this we remark that the level curves on isobaric sur- 

 faces and the isobaric curves on level surfaces belong to one family, the curves of 

 intersection between isobaric and level surfaces. The level curves on an isobaric 

 surface and the isobaric curves on a level surface from about the same height in the 

 atmosphere will therefore resemble each other. Further, the standard isobaric 

 surfaces of pressures 1000, 900, 800, 700, 600, 500, 400, and 300 m-bars are nearly 

 in the levels of o, 1000, 2000, 3000, 4000, 5000, 7000, and 9000 dynamic meters, and 

 therefore only a small correction is required to [change the level curves of these 

 isobaric surfaces into isobaric curves at the correspondingjevels. 



The principles for finding these corrections are easily seen. The isobaric curve 

 700 m-bars in the level surface 3000 dynamic meters is identical with the given 

 level curve 3000 dynamic meters on the isobaric surface 700 m-bars. The isobaric 



