FIELDS OF PRESSURE AND MASS IN THE ATMOSPHERE. 95 



curve 705 m-bars will run where the sheet of air between the isobaric and the 

 level surface exerts the pressure of 5 m-bars. In order to exert this pressure the 

 sheet must have the thickness of 56 dynamic meters if it has the temperature of 

 C, the thickness of 58 dynamic meters if it has the temperature of io C, and so 

 on. This is seen at once from tables iom and 12 m. The required isobaric curve 

 of 705 m-bars will thus coincide with the given level curve of 3056 dynamic meters 

 where the sheet has the temperature of o C, with the given level curve 3058 dy- 

 namic meters where the sheet has the temperature of io C, and so on. These tem- 

 peratures and the level curves being given, the isobaric curves can thus be drawn. 

 To avoid the laborious use of tables 10 m and 12 m, table 17 m has been derived 

 from them ; as one argument appears the pressures along any isobaric curve to be 

 drawn, and as the other the virtual temperatures in the given isobaric surface. 

 These are always known (see example 1, column 9, table J, p. 70, example 2, 

 column 10, table K, p. 74). To these temperatures at the surface will correspond 

 a definite average temperature of the sheet if we make the common supposition of 

 a 'fall of temperature of 0.5 C. for every 100 dynamic meters of height. On 

 account of the smallness of the reductions a greater accuracy than that obtained 

 under this simple supposition will never be required. Using this supposition, the 

 tabulated numbers are derived from tables iom and 12 m. They indicate with 

 which level curves the required isobaric curves should coincide. Using these tables 

 and the given topographic chart and temperature chart for the isobaric surfaces } 

 the required isobaric charts can be drawn with great ease. 



68. Correction of Charts for Lower Levels and Construction of Charts for 

 Higher Levels by Means of Observations Obtained from Ascents. If results 

 from simultaneous ascents in the air were available in sufficient number, charts of 

 absolute and of mutual topography, or of absolute pressure and pressure differences, 

 could be drawn directly and independently of each other for every level. But as 

 long as these ascents remain comparatively rare, it will be advisable first to draw 

 all charts which can be obtained by extrapolation from the stations at the earth's 

 surface as completely as possible. 



This being done, our first task will be to correct the charts according to the 

 absolute values obtained from the ascents. This is easily done for charts of abso- 

 lute topography or of absolute pressure. The values obtained from the ascents are 

 noted on the charts, and the whole set of curves displaced or changed so as to suit 

 these values. As a rule this is easily done without any noticeable change in the 

 qualitative course of the curves. These corrections have been made on the charts 

 of figs. 19 and 22. 



Greater difficulty will be found in correcting charts of mutual topography or of 

 pressure differences, because their curves have a very complicated course, evidently 

 in great measure depending upon the topography of the land and the distribution 

 of land and sea. It is not easy to see how to change the course of such curves so 

 as to suit the small number of correct values obtained by the ascents. In the ex- 

 amples worked out below we have therefore desisted from making this correction. 



