EXAMPLES OF ATMOSPHERIC MOTIONS. 171 



double lines, consisting of a thick line and a thin one, give the relative topography 

 of the 900 m-bar surface relatively to the 1000 m-bar surface, and thus the average 

 specific volume of the air in the sheet between these two surfaces. The thin line is 

 on the side where the sheet is thinner. All lines are stippled where they have their 

 course below the ground. It will be seen that the 1000 m-bar surface has a strong 

 depression, going down to 100 dynamic meters below sea-level in southern Minnesota, 

 with a secondary depression in Colorado. The great area of depression is surrounded 

 by high areas situated in New England, in Montana and the adjacent parts of 

 Canada, and on the southern part of the coast of California. Another depression 

 is situated farther north on the Pacific coast. 



Plate XXXVI gives the representation of the observed wind-directions in the 

 common way by arrows. The corresponding numbers, according to the dial of fig. 32, 

 are also inscribed, and another set of numbers give the wind-intensities in meters per 

 second. A glance at the arrows at once shows the unfortunate consequences of the 

 observation of only eight wind-directions. If the lines of flow were drawn strictly 

 tangential to the arrows they would get polygonal form, with a great number of 

 lines of convergence and of divergence separating from each other the areas of 

 different wind-directions. It must therefore be highly recommended to observe 

 at least double the number of wind-directions. Provisionally we can only remove 

 the discontinuities in the drawing of the isogonal curves or the lines of flow by 

 eye-measure. 



Plate XXXVII gives the continuous representation of the motion by isogonal 

 curves and curves of equal wind-intensity. Plate XXXVIII gives the same repre- 

 sentation by lines of flow and intensity-curves. The isogonal curves have a remark- 

 ably simple course: only two singular points appear, one in southern Minnesota 

 and one in California the former positive, the latter negative. The lines of flow 

 show a marked point of convergence in southern Minnesota, near the point of the 

 lowest depression, and several lines of convergence which run into this point. A line 

 of divergence connects the two high areas in Montana and California, and this line 

 has a neutral point where the isogonal curves had the negative singular point. The 

 lines of flow make a very striking bend in order to go around instead of across the 

 Allegheny mountains. While the lines of flow have a relatively simple course, the 

 distribution of wind-intensity is very irregular, with a great number of maxima and 

 minima. North of the cyclonic center winds go up to 28 meters per second. 



Chart XXXVIII is drawn upon a blank surface which gives the topography of 

 the land greatly idealized. By the method of section 181 we have derived from it the 

 chart of plate XXXIX, which gives the vertical velocity at the ground. The shaded 

 areas on the windward slopes are those of ascending motion, the unshaded ones on 

 the leeward slopes those of descending motion. The greatest vertical velocities 

 amount to 20 cm. per second. The fact that higher values are never reached is of 

 course due to the idealization of topography. The true local values may be much 

 greater, while our chart gives only the average values for greater areas. 



From either of the two plates XXXVII and XXXVIII we can derive by the 

 solenoidal condition the free vertical motion, which is represented by the chart of 



