THE FORCED VERTICAL MOTION AT THE BOUNDING SURFACES. 



137 



When we use the average relation between pressure and height given in Statics, 

 table A, p. 29, we get the tables O. The first gives the temperature at points where 

 the contour-lines of the topographic map are cut by the required curves of equal 

 density, while the second gives the height of the points where the required density- 

 curves cut the given isotherms. As density is proportional to pressure, and as 

 pressure at a place will as a rule differ only a small percentage from its average 



Tables N. 



value, the use of this simpler method will generally give only a small percentage of 

 errors of the values of density. This will be of no importance when the general 

 inaccuracy of the observations of velocity is considered. 



Instead of drawing a chart of temperature at the ground, we can take an 

 average value of the temperature, and draw the density-chart by use of that column 

 in table O, b, which corresponds to this temperature. 



A. Temperature at points where isopyenic 

 curves cut contour-lines. 



Tables O. 



B. Height of points where isopyenic curves cut isotherms. 



We can even simplify still further, and neglect also the variations of tempera- 

 ture. We then consider density as given only by the height above sea-level, for 

 instance, by the column for o C of table O, b. 



In this case we should then always use the same density-chart, which could 

 be derived from the contour-lines of the topographic chart by use of the numbers 

 in this column. 



